ceph: fix incorrect kmalloc size of pagevec mempool

[ Upstream commit 03230edb0bd831662a7c08b6fef66b2a9a817774 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>