ext4: wait for writeback to complete while making pages writable

In order to stabilize pages during disk writes, ext4_page_mkwrite must
wait for writeback operations to complete before making a page
writable.  Furthermore, the function must return locked pages, and
recheck the writeback status if the page lock is ever dropped.  The
"someone could wander in" part of this patch was suggested by Chris
Mason.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>