GFS2: Make ->write_inode() really write

The GFS2 ->write_inode function should be more aggressive at writing
back to the filesystem. This adopts the XFS system of returning
-EAGAIN when the writeback has not been completely done. Also, we
now kick off in-place writeback when called with WB_SYNC_NONE,
but we only wait for it and flush the log when WB_SYNC_ALL is
requested.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>