mm/mempool.c: add missing parameter description

The kernel-doc for mempool_init function is missing the description of the
pool parameter.  Add it.

Link: http://lkml.kernel.org/r/1532336274-26228-1-git-send-email-rppt@linux.vnet.ibm.com


Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>