(pre-stable) ext4: fix freeze deadlock under IO

BugLink: http://bugs.launchpad.net/bugs/595489

Commit 6b0310fb

 caused a regression resulting in deadlocks
when freezing a filesystem which had active IO; the vfs_check_frozen
level (SB_FREEZE_WRITE) did not let the freeze-related IO syncing
through.  Duh.

Changing the test to FREEZE_TRANS should let the normal freeze
syncing get through the fs, but still block any transactions from
starting once the fs is completely frozen.

I tested this by running fsstress in the background while periodically
snapshotting the fs and running fsck on the result.  I ran into
occasional deadlocks, but different ones.  I think this is a
fine fix for the problem at hand, and the other deadlocky things
will need more investigation.

Reported-by: Phillip Susi <psusi@cfl.rr.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
(cherry-picked from commit 437f88cc031ffe7f37f3e705367f4fe1f4be8b0f upstream)
Acked-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Steve Conklin <steve.conklin@canonical.com>
Acked-by: Andy Whitcroft <andy.whitcroft@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>