For filesystems such as nilfs2 and xfs that use block_page_mkwrite, modify that
function to wait for pending writeback before allowing the page to become
writable.  This is needed to stabilize pages during writeback for those two
filesystems.
Signed-off-by: NDarrick J. Wong <djwong@us.ibm.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>