fs-verity: use kmap_local_page() instead of kmap()

Convert the use of kmap() to its recommended replacement
kmap_local_page().  This avoids the overhead of doing a non-local
mapping, which is unnecessary in this case.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Link: https://lore.kernel.org/r/20220818224010.43778-1-ebiggers@kernel.org