btrfs-progs: mkfs: check the status of file at mkfs

btrfs-progs: mkfs: check the status of file at mkfs

Currently, only the status of block devices is checked at mkfs,
but we should also check for regular files whether they are already
formatted or mounted to prevent overwrite accidentally.

Device status is checked by test_dev_for_mkfs().
The part which is not related to block device is split from this
and used for both block device and regular file.

Signed-off-by: Tomohiro Misono <misono.tomohiro@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>