btrfs: assert that relocation is protected with sb_start_write()

Relocation of a data block group creates ordered extents. They can cause
a hang when a process is trying to thaw the filesystem.

We should have called sb_start_write(), so the filesystem is not being
frozen. Add an ASSERT to check it is protected.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>