fscrypt: clean up comments in bio.c

The file comment in bio.c is almost completely irrelevant to the actual
contents of the file; it was originally copied from crypto.c.  Fix it
up, and also add a kerneldoc comment for fscrypt_decrypt_bio().

Link: https://lore.kernel.org/r/20210909190737.140841-1-ebiggers@kernel.org


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