libmount: run btrfs subvol checks for "auto" fs type

libmount: run btrfs subvol checks for "auto" fs type

It is possible to mount btrfs using "auto" keyword in fstab. In such
case, btrfs specific checks are skipped. Run them for "auto" as well.

Looking at the code, it is a safe approach. In case of btrfs, it will do
what is needed, in case of no btrfs, btrfs_get_default_subvol_id() will
fail, and the rest of the code is skipped.

How to reproduce:
See reproducer in 2cd28fc and replace fstab line by
	echo "/dev/loop0 $PWD/btrfs_mnt btrfs auto 0 0" >>/etc/fstab

Current behavior of second "mount -a":
	mount: /dev/loop0 is already mounted or /root/btrfs_mnt busy
	       /dev/loop0 is already mounted on /root/btrfs_mnt

Testcases for btrfs and ext4:
	truncate -s1G btrfs_test.img
	truncate -s1G ext4_test.img
	mkdir -p btrfs_mnt
	mkdir -p ext4_mnt
	/sbin/mkfs.btrfs -f -d single -m single ./btrfs_test.img
	/sbin/mkfs.ext4 ./ext4_test.img
	losetup /dev/loop0 $PWD/btrfs_test.img
	losetup /dev/loop1 $PWD/ext4_test.img
	echo "/dev/loop0 $PWD/btrfs_mnt auto defaults 0 0" >>/etc/fstab
	echo "/dev/loop1 $PWD/ext4_mnt auto defaults 0 0" >>/etc/fstab
	./mount -a
	./mount -a
	umount btrfs_mnt
	umount ext4_mnt
	sed -i "/\/dev\/loop[01]/d" /etc/fstab
	losetup -d /dev/loop0
	losetup -d /dev/loop1
	rm btrfs_test.img
	rm ext4_test.img
	rmdir btrfs_mnt
	rmdir ext4_mnt

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>