vfs: dodge smp_mb in break_lease and break_deleg in the common case

vfs: dodge smp_mb in break_lease and break_deleg in the common case

These inlines show up in the fast path (e.g., in do_dentry_open()) and
induce said full barrier regarding i_flctx access when in most cases the
pointer is NULL.

The pointer can be safely checked before issuing the barrier, dodging it
in most cases as a result.

It is plausible the consume fence would be sufficient, but I don't want
to go audit all callers regarding what they before calling here.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240806172846.886570-1-mjguzik@gmail.com
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>