This patch closes a timing window in which two processes compete
and overlap in the execution of do_xmote for the same glock:

             Process A                              Process B
   ------------------------------------   -----------------------------
1. Grabs gl_lockref and calls do_xmote
2.                                        Grabs gl_lockref but is blocked
3. Sets GLF_INVALIDATE_IN_PROGRESS
4. Unlocks gl_lockref
5.                                        Calls do_xmote
6. Call glops->go_sync
7. test_and_clear_bit GLF_DIRTY
8. Call gfs2_log_flush                    Call glops->go_sync
9. (slow IO, so it blocks a long time)    test_and_clear_bit GLF_DIRTY
                                          It's not dirty (step 7) returns
10.                                       Tests GLF_INVALIDATE_IN_PROGRESS
11.                                       Calls go_inval (rgrp_go_inval)
12.                                       gfs2_rgrp_relse does brelse
13.                                       truncate_inode_pages_range
14.                                       Calls lm_lock UN

In step 14 we've just told dlm to give the glock to another node
when, in fact, process A has not finished the IO and synced all
buffer_heads to disk and make sure their revokes are done.

This patch fixes the problem by changing the GLF_INVALIDATE_IN_PROGRESS
to use test_and_set_bit, and if the bit is already set, process B just
ignores it and trusts that process A will do the do_xmote in the proper
order.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>