LU-12753 tests: wait for mds2 recovery in sanity 278

LU-12753 tests: wait for mds2 recovery in sanity 278

At the end of sanity.sh test_278() the mds2 facet is remounted.
However, test_300a() which runs immediately afterward expects
all MDTs to be available, and occasionally fails creating a
fully-striped directory with:

    stripe_count is 1, expect 2

because the mds2 facet has not finished mounting at mkdir time.

Add a wait at the end of test_278() to ensure that mds2 has finished
mounting so that it doesn't affect any later tests.

Test-Parameters: trivial envdefinitions=ONLY=270-300 mdscount=1 mdtcount=2
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I9421101be6394288ccd4ce5777d118523a3ebbe5
Reviewed-on: https://review.whamcloud.com/36167
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>