fs/buffer.c: use b_folio for fsverity work

Use b_folio now that it exists.  This removes an unnecessary call to
compound_head().  No actual change in behavior.

Link: https://lore.kernel.org/r/20230224232530.98440-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>