ocfs2_grab_pages_for_write() may return -EAGAIN if write context type is
mmap and it could not lock the target page.  In this case, we exit with
no error and no target page.  And then trigger the caller page_mkwrite()
to retry.

Since there are other caller types, e.g.  buffer and direct io, make the
return value handling more clear.

Link: https://lkml.kernel.org/r/20211206065051.103353-1-joseph.qi@linux.alibaba.comSigned-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Gang He <ghe@suse.com>
Cc: Jun Piao <piaojun@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>