libmount: handle btrfs default subvolume mount

libmount: handle btrfs default subvolume mount

When mounting btrfs volume without subvol= and subvolid=, and the
btrfs volume has default subvolume defined, mount() mounts the default
subvolume and not the volume root as other filesystems do.

To handle this situation correctly (for example for "mount -a"),
libmount has to be capable to detect default subvolume.

Add btrfs.c and btrfs.h that implement needed functions.

This patch adds mnt_table_find_target_with_option() to the library API.

Known problems not covered by this patch:

- Use of subvolid= in fstab is not yet handled.

- Use of type auto in combination with subvol= in fstab is not yet
  handled.

- Use of btrfs in loop devices, where image file is specified in fstab is
  not yet handled (use of /dev/loop0 in fstab works).

- If fstab uses subvol=, and subvol path changes since last "mount -a",
  subsequent "mount -a" will not recognize that it is already mounted,
  and it will attempt to mount it second time. To fix it, libmount should
  remember subvolid in time of mount (subvolid is unique for the
  subvolume, subvol is not).

- mountinfo contains subvol and subvolid since kernel 4.2. Before kernel
  4.2, there is no reasonable way to solve this situation. (One would
  create temporary mount point, mount the default, call needed ioctl() to
  determine what was mounted, deduce the default subvolume, compare it
  with subvolume of mounted volume, unmount and return result.)

How to reproduce:
	truncate -s1G btrfs_test.img
	mkdir -p btrfs_mnt
	/sbin/mkfs.btrfs -f -d single -m single ./btrfs_test.img
	mount -o loop btrfs_test.img btrfs_mnt
	pushd .
	cd btrfs_mnt
	mkdir -p d0/dd0/ddd0
	cd d0/dd0/ddd0
	touch file{1..5}
	btrfs subvol create s1
	cd s1
	touch file{1..5}
	mkdir -p d1/dd1/ddd1
	cd d1/dd1/ddd1
	btrfs subvol create s2
	rid=$(btrfs inspect rootid s2)
	echo new default $rid
	btrfs subvol get-default .
	btrfs subvol set-default $rid .
	popd
	umount btrfs_mnt
	losetup /dev/loop0 $PWD/btrfs_test.img
	echo "/dev/loop0 $PWD/btrfs_mnt btrfs defaults 0 0" >>/etc/fstab
	mount -a
	mount -a
	umount btrfs_mnt
	sed -i "/\/dev\/loop0/d" /etc/fstab
	losetup -d /dev/loop0
	rm btrfs_test.img
	rmdir btrfs_mnt

Current behavior:
	mount: /dev/loop0 is already mounted or /root/btrfs_mnt busy
	       /dev/loop0 is already mounted on /root/btrfs_mnt

Expected behavior is to ignore already mounted FS.

[kzak@redhat.com: - make 'var' optional for mnt_table_find_target_with_option(),
                  - add mnt_table_find_target_with_option() to symbols table and docs
                  - add "btrfs" string between supported debug modes
                  - minor coding style changes]

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>
Cc: David Štěrba <dsterba@suse.cz>
Signed-off-by: Karel Zak <kzak@redhat.com>