fscrypt: add "fscrypt_" prefix to fname_encrypt()

fname_encrypt() is a global function, due to being used in both fname.c
and hooks.c.  So it should be prefixed with "fscrypt_", like all the
other global functions in fs/crypto/.

Link: https://lore.kernel.org/r/20200120071736.45915-1-ebiggers@kernel.org


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