ocfs2: ocfs2_write_begin_nolock() should handle len=0

commit 8379e7c4 upstream.

Bug introduced by mainline commit e7432675
The bug causes ocfs2_write_begin_nolock() to oops when len=0.

Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>