ubi: fastmap: Allocate memory with GFP_NOFS in ubi_update_fastmap

Function ubi_update_fastmap could be called in IO context, for example:
 ubifs_writepage
  do_writepage
   ubifs_jnl_write_data
    write_head
     ubifs_wbuf_write_nolock
      ubifs_leb_write
       ubi_leb_write
        ubi_eba_write_leb
	 try_write_vid_and_data
	  ubi_wl_get_peb
	   ubi_update_fastmap
	    erase_block

So it's better to allocate memory with GFP_NOFS mode, in case waiting
page writeback(dead loop).

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>