btrfs-progs: fix find_mount_root() to handle duplicated mount point correctly

btrfs-progs: fix find_mount_root() to handle duplicated mount point correctly

Original find_mount_root() will use the first mount point match and
return it.
It was OK until the following commit, which will also check the fstype:
de22c28ef31d9721606ba059 btrfs-progs: Check fstype in find_mount_root()

With fstype check, we should check the last match, not only the first
one.
Or the following mount will not pass the find_mount_root():
/dev/sdc on /mnt/test type ext4 (rw,relatime,data=ordered)
/dev/sdb on /mnt/test type btrfs (rw,relatime,space_cache)

This patch will use the last match to do the fstype check.

Reported-by: Remco Hosman <remco@yerf-it.nl>
Signed-off-by: Remco Hosman <remco@yerf-it.nl>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Omar Sandoval <osandov@osandov.com>
Signed-off-by: David Sterba <dsterba@suse.cz>