ubifs: Fix FS_IOC_SETFLAGS unexpectedly clearing encrypt flag

UBIFS's implementation of FS_IOC_SETFLAGS fails to preserve existing
inode flags that aren't settable by FS_IOC_SETFLAGS, namely the encrypt
flag.  This causes the encrypt flag to be unexpectedly cleared.

Fix it by preserving existing unsettable flags, like ext4 and f2fs do.

Test case with kvm-xfstests shell:

    FSTYP=ubifs KEYCTL_PROG=keyctl
    . fs/ubifs/config
    . ~/xfstests/common/encrypt
    dev=$(__blkdev_to_ubi_volume /dev/vdc)
    ubiupdatevol -t $dev
    mount $dev /mnt -t ubifs
    k=$(_generate_session_encryption_key)
    mkdir /mnt/edir
    xfs_io -c "set_encpolicy $k" /mnt/edir
    echo contents > /mnt/edir/file
    chattr +i /mnt/edir/file
    chattr -i /mnt/edir/file

With the bug, the following errors occur on the last command:

    [   18.081559] fscrypt (ubifs, inode 67): Inconsistent encryption context (parent directory: 65)
    chattr: Operation not permitted while reading flags on /mnt/edir/file

Fixes: d475a507 ("ubifs: Add skeleton for fscrypto")
Cc: <stable@vger.kernel.org> # v4.10+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/ioctl.c

@@ -113,7 +113,8 @@ static int setflags(struct inode *inode, int flags)
 	if (err)
 		goto out_unlock;
 
-	ui->flags = ioctl2ubifs(flags);
+	ui->flags &= ~ioctl2ubifs(UBIFS_SUPPORTED_IOCTL_FLAGS);
+	ui->flags |= ioctl2ubifs(flags);
 	ubifs_set_inode_flags(inode);
 	inode->i_ctime = current_time(inode);
 	release = ui->dirty;