ceph: fix incorrect kmalloc size of pagevec mempool

The kmalloc size of pagevec mempool is incorrectly calculated.
It misses the size of page pointer and only accounts the number for the array.

Fixes: a0102bda

 ("ceph: move sb->wb_pagevec_pool to be a global mempool")
Signed-off-by: ethanwu <ethanwu@synology.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>