btrfs-progs: mkfs: do not try to register non-block-devices

btrfs-progs: mkfs: do not try to register non-block-devices

A mkfs on a regular file will try to call the DEV_SCAN ioctl that will
take the loop through the kernel and fail:

 ERROR: device scan failed './test.img' - Block device required

If a user without permission to open the control device tries to mkfs a
regular file, an error message is printed:

 failed to open /dev/btrfs-control skipping device registration: Permission denied

So we should not try to pass a non-block-device besides that this makes
the --quiet option more quiet.

Signed-off-by: David Sterba <dsterba@suse.cz>