fscrypt: move fscrypt_operations declaration to fscrypt_supp.h

Filesystems now only define their fscrypt_operations when they are
compiled with encryption support, so move the fscrypt_operations
declaration from fscrypt.h to fscrypt_supp.h.

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