fscrypt: use FSCRYPT_* definitions, not FS_*

fscrypt: use FSCRYPT_* definitions, not FS_*

Update fs/crypto/ to use the new names for the UAPI constants rather
than the old names, then make the old definitions conditional on
!__KERNEL__.

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