fsverity: optimize fsverity_prepare_setattr() on non-verity files

Make fsverity_prepare_setattr() an inline function that does the
IS_VERITY() check, then (if needed) calls __fsverity_prepare_setattr()
to do the real work.  This reduces the overhead on non-verity files.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Dave Chinner <dchinner@redhat.com>
Link: https://lore.kernel.org/r/20221214224304.145712-3-ebiggers@kernel.org