f2fs: clean up f2fs_may_encrypt()

Merge below two conditions into f2fs_may_encrypt() for cleanup
- IS_ENCRYPTED()
- DUMMY_ENCRYPTION_ENABLED()

Check IS_ENCRYPTED(inode) condition in f2fs_init_inode_metadata()
is enough since we have already set encrypt flag in f2fs_new_inode().

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>