ext4: BUG_ON fix: check if page has buffers before calling page_buffers()

We need to make check if a page does not have buffes by checking
page_has_buffers(page) before calling page_buffers(page) in
ext4_writepage().  Otherwise page_buffers() could throw a BUG_ON.

Thanks also to Markus Trippelsdorf and Avinash Kurup who also reported
the problem.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reported-by: Sedat Dilek <sedat.dilek@googlemail.com>
Tested-by: Sedat Dilek <sedat.dilek@googlemail.com>