fs/pipe: Fix pipe_occupancy() with 16-bit indexes

mainline inclusion
from mainline-v6.14-rc6
category: bugfix

The pipe_occupancy() logic implicitly relied on the natural unsigned
modulo arithmetic in C, but that doesn't work for the new 'pipe_index_t'
case, since any arithmetic will be done in 'int' (and here we had also
made it 'unsigned int' due to the function call boundary).

So make the modulo arithmetic explicit by casting the result to the
proper type.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Swapnil Sapkal <swapnil.sapkal@amd.com>
Cc: Alexey Gladkov <legion@kernel.org>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Link: https://lore.kernel.org/all/CAHk-=wjyHsGLx=rxg6PKYBNkPYAejgo7=CbyL3=HGLZLsAaJFQ@mail.gmail.com/
Fixes: 3d25216 ("fs/pipe: Read pipe->{head,tail} atomically outside pipe->mutex")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit c27c66a)
Signed-off-by: Wentao Guan <guanwentao@uniontech.com>
Change-Id: I0d0bb4c768cd56ae5054a972a70d432a1099fd83

Author: torvalds

include/linux/pipe_fs_i.h

@@ -192,7 +192,7 @@ static inline bool pipe_empty(unsigned int head, unsigned int tail)
  */
 static inline unsigned int pipe_occupancy(unsigned int head, unsigned int tail)
 {
-	return head - tail;
+	return (pipe_index_t)(head - tail);
 }
 
 /**