fscrypt: move fscrypt_symlink_data to fscrypt_private.h

Now that all filesystems have been converted to use the symlink helper
functions, they no longer need the declaration of 'struct
fscrypt_symlink_data'.  Move it from fscrypt.h to fscrypt_private.h.

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