gfs2: Convert remaining kmap_atomic calls to kmap_local_page

Replace the remaining instances of kmap_atomic() ... kunmap_atomic()
with kmap_local_page() ... kunmap_local().

In gfs2_write_buf_to_page(), we can call flush_dcache_page() after
unmapping the page.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>