fscrypto: rename completion callbacks to reflect usage

fscrypt_complete() was used only for data pages, not for all
encryption/decryption.  Rename it to page_crypt_complete().

dir_crypt_complete() was used for filename encryption/decryption for
both directory entries and symbolic links.  Rename it to
fname_crypt_complete().

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>