gfs2: Simplify gfs2_read_super

Use submit_bio_wait() instead of hand-rolling our own synchronous
wait.  Also allocate the BIO on the stack since we're not deep in
the call stack at this point.

There's no need to kmap the page, since it isn't allocated from HIGHMEM.
Turn the GFP_NOFS allocation into GFP_KERNEL; if the page allocator
enters reclaim, we cannot be called as the filesystem has not yet been
initialised and so has no pages to reclaim.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>