Btrfs: clean up error handling in mount_subvol()
Btrfs: clean up error handling in mount_subvol()

In preparation for new functionality in mount_subvol(), give it
ownership of subvol_name and tidy up the error paths.

Reviewed-by: David Sterba <a2ce9d316ca04d17b520237d2846a218b8284e52@suse.cz>
Signed-off-by: Omar Sandoval <888fed946b7dbf3db82157abfe4a5af66080e99e@osandov.com>
Signed-off-by: Chris Mason <e2e8c5702d8ed5f565988a1cf5bb35c9ce199e5e@fb.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs