gfs2: use bdev_rw_virt in gfs2_read_super

Switch gfs2_read_super to allocate the superblock buffer using kmalloc
which falls back to the page allocator for PAGE_SIZE allocation but
gives us a kernel virtual address and then use bdev_rw_virt to perform
the synchronous read into it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20250507120451.4000627-11-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>