[GFS2] Revise readpage locking

The previous attempt to fix the locking in readpage failed due
to the use of a "try lock" which resulted in occasional high
cpu usage during testing (due to repeated tries) and also it
did not resolve all the ordering problems wrt the transaction
lock (although it did solve all the inode lock ordering problems).

This patch avoids the problem by unlocking the page and getting the
locks in the correct order. This means that we have to retest the
page to ensure that it hasn't changed when we relock the page.

This now passes the tests which were previously failing.

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