nvme-multipath: revalidate nvme_ns_head gendisk in nvme_validate_ns

When CONFIG_NVME_MULTIPATH is set, only the hidden gendisk associated
with the per-controller ns is run through revalidate_disk when a
rescan is triggered, while the visible blockdev never gets its size
(bdev->bd_inode->i_size) updated to reflect any capacity changes that
may have occurred.

This prevents online resizing of nvme block devices and in extension of
any filesystems atop that will are unable to expand while mounted, as
userspace relies on the blockdev size for obtaining the disk capacity
(via BLKGETSIZE/64 ioctls).

Fix this by explicitly revalidating the actual namespace gendisk in
addition to the per-controller gendisk, when multipath is enabled.

Signed-off-by: Anthony Iliopoulos <ailiopoulos@suse.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>