btrfs: make file clone aware of fatal signals

Indeed this just make the behavior similar to xfs when process has
fatal signals pending, and it'll make fstests/generic/298 happy.

Signed-off-by: Wang Xiaoguang <wangxg.fnst@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>