vfs: elide smp_mb in iversion handling in the common case

vfs: elide smp_mb in iversion handling in the common case

According to bpftrace on these routines most calls result in cmpxchg,
which already provides the same guarantee.

In inode_maybe_inc_iversion elision is possible because even if the
wrong value was read due to now missing smp_mb fence, the issue is going
to correct itself after cmpxchg. If it appears cmpxchg wont be issued,
the fence + reload are there bringing back previous behavior.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240815083310.3865-1-mjguzik@gmail.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>