btrfs: use GFP_KERNEL in btrfs_init_dev_replace_tgtdev

The function is called from ioctl context and we don't hold any locks
that take part in writeback. Right now it's only fs_info::volume_mutex.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>