LU-10843 mgs: allow snapshot after MGS remount

If the MGS is unmounted/mounted without the MDS reconnecting,
the fsdb FSNAME-barrier would not be created.

This change allows mgs_barrier_freeze (called from snapshot_create)
to attempt the create the required fsdb.

This adds a test to sanity-lsnapshot.sh for this issue.

Test-Parameters: testlist=sanity-lsnapshot mgtfilesystemtype=zfs ostfilesystemtype=zfs mdtfilesystemtype=zfs combinedmdsmgs=false ostcount=2 standalonemgs=true
Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I2432cc0bdaddb07f024744065ca2ced77288fd7b
Reviewed-on: https://review.whamcloud.com/31779


Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>