mm/zpool: prevent zbud/zsmalloc from unloading when used

mm/zpool: prevent zbud/zsmalloc from unloading when used

Add try_module_get() to zpool_create_pool(), and module_put() to
zpool_destroy_pool().  Without module usage counting, the driver module(s)
could be unloaded while their pool(s) were active, resulting in an oops
when zpool tried to access them.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Cc: Seth Jennings <sjennings@variantweb.net>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Weijie Yang <weijie.yang@samsung.com>
Reviewed-by: Bob Liu <bob.liu@oracle.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Rik van Riel <riel@redhat.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>