ubifs: Add support for FS_ENCRYPT_FL

Make the FS_IOC_GETFLAGS ioctl on ubifs return the FS_ENCRYPT_FL flag on
encrypted files, like ext4 and f2fs do.

Also make this flag be ignored by FS_IOC_SETFLAGS, like ext4 and f2fs
do, since it's a recognized flag but is not directly settable.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>