btrfs: tracepoints: fix sleep while in atomic context in btrfs_sync_file()

The trace event btrfs_sync_file() is called in an atomic context (all trace
events are) and its call to dput(), which is needed due to the call to
dget_parent(), can sleep, triggering a kernel splat.

This can be reproduced by enabling the trace event and running btrfs/056
from fstests for example. The splat shown in dmesg is the following:

   [84853.995109] BUG: sleeping function called from invalid context at fs/dcache.c:970
   [84853.996457] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 32773, name: xfs_io
   [84853.997858] preempt_count: 2, expected: 0
   [84853.998647] RCU nest depth: 0, expected: 0
   [84853.999403] Preemption disabled at:
   [84853.999404] [<0000000000000000>] 0x0
   [84854.000708] CPU: 0 UID: 0 PID: 32773 Comm: xfs_io Tainted: G        W           7.1.0-rc1-btrfs-next-232+ #1 PREEMPT(full)
   [84854.000710] Tainted: [W]=WARN
   [84854.000711] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.2-0-gea1b7a073390-prebuilt.qemu.org 04/01/2014
   [84854.000712] Call Trace:
   [84854.000714]  <TASK>
   [84854.000716]  dump_stack_lvl+0x56/0x80
   [84854.000719]  __might_resched.cold+0xd6/0x10f
   [84854.000722]  dput.part.0+0x24/0x110
   [84854.000728]  trace_event_raw_event_btrfs_sync_file+0x75/0x140 [btrfs]
   [84854.000809]  btrfs_sync_file+0x1ed/0x530 [btrfs]
   [84854.000887]  ? __handle_mm_fault+0x8ae/0xed0
   [84854.000889]  btrfs_do_write_iter+0x172/0x210 [btrfs]
   [84854.000951]  vfs_write+0x21f/0x450
   [84854.000954]  __x64_sys_pwrite64+0x8d/0xc0
   [84854.000956]  ? do_user_addr_fault+0x20c/0x670
   [84854.000959]  do_syscall_64+0x60/0xf20
   [84854.000962]  ? clear_bhb_loop+0x60/0xb0
   [84854.000964]  entry_SYSCALL_64_after_hwframe+0x76/0x7e

So stop using dget_parent() and dput() and access the parent dentry
directly as dentry->d_parent. This is also what ext4 is doing in
its equivalent trace event ext4_sync_file_enter().

Fixes: a85b46d ("btrfs: tracepoints: get correct superblock from dentry in event btrfs_sync_file()")
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>

Author: fdmanana

include/trace/events/btrfs.h

@@ -771,10 +771,8 @@ TRACE_EVENT(btrfs_sync_file,
 	TP_fast_assign(
 		struct dentry *dentry = file_dentry(file);
 		struct inode *inode = file_inode(file);
-		struct dentry *parent = dget_parent(dentry);
-		struct inode *parent_inode = d_inode(parent);
+		struct inode *parent_inode = d_inode(dentry->d_parent);
 
-		dput(parent);
 		TP_fast_assign_fsid(btrfs_sb(inode->i_sb));
 		__entry->ino		= btrfs_ino(BTRFS_I(inode));
 		__entry->parent		= btrfs_ino(BTRFS_I(parent_inode));