On filesystems with a block size smaller than the page size,
gfs2_find_jhead can split a page across two bios (for example, when
blocks are not allocated consecutively).  When that happens, the first
bio that completes will unlock the page in its bi_end_io handler even
though the page hasn't been read completely yet.  Fix that by using a
chained bio for the rest of the page.

While at it, clean up the sector calculation logic in
gfs2_log_alloc_bio.  In gfs2_find_jhead, simplify the disk block and
offset calculation logic and fix a variable name.

Fixes: f4686c26 ("gfs2: read journal in large chunks")
Cc: stable@vger.kernel.org # v5.2+
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>