ubifs: Remove #ifdef around CONFIG_FS_ENCRYPTION

ifdefs reduce readablity and compile coverage. This removes the ifdefs
around CONFIG_FS_ENCRYPTION by using IS_ENABLED and relying on static
inline wrappers. A new static inline wrapper for setting sb->s_cop is
introduced to allow filesystems to unconditionally compile in their
s_cop operations.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Richard Weinberger <richard@nod.at>