This patch fixes a mmap_truncate bug which was found by ocfs2 test suite.

In an ocfs2 cluster more than 1 node, run program mmap_truncate, which races
mmap writes and truncates from multiple processes. While the test is
running, a stat from another node forces writeout, causing an oops in
ocfs2_get_block() because it sees a buffer to write which isn't allocated.

This patch fixed the bug by clear dirty and uptodate bits in buffer, leave
the buffer unmapped and return.

Fix is suggested by Mark Fasheh, and I code up the patch.
Signed-off-by: Coly Li <coyli@suse.de>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>