fs, fscrypt: only define ->s_cop when FS_ENCRYPTION is enabled

Now that filesystems only set and use their fscrypt_operations when they
are built with encryption support, we can remove ->s_cop from
'struct super_block' when FS_ENCRYPTION is disabled.  This saves a few
bytes on some kernels and also makes it consistent with ->i_crypt_info.

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