LU-10235 mdt: mdt_create: check EEXIST without lock

LU-10235 mdt: mdt_create: check EEXIST without lock

mkdir() currently gets a write lock on the parent even if the new
directory already exists.

This patch adds an initial lookup of the new directory without a DLM
lock so that other clients do not need to cancel their DLM lock if the
"new" directory already exists, but will continue as usual if directory
did not exist.

There is a small race window that child was created by others after our
check and before locking parent, but this can be detected later during
index insert.

Performance change on two haswell 16-core VMs with ib, mean values of
mpirun -n 8 ./mdtest -D -i 8 -I 1000

test environment | directory creation | tree creation
local, no patch  | 1725/s             | 769/s
local, patch     | 1821/s             | 788/s
remote, no patch | 1729/s             | 772/s
remote, patch    | 1687/s             | 787/s

The differences are of the order of the noise here, with all mkdirs
being effective.

If directories exist, some simple stress on four nodes shows intended
improvements:
clush -w vm[0-3] 'seq 0 10000 |
    xargs -P 7 -I{} sh -c "(({}%3==0)) &&
        mkdir /mnt/lustre/testdir/foo 2>/dev/null ||
        stat /mnt/lustre/testdir > /dev/null"'

with patch: 10s
without patch: 19s
(the difference grows exponentially with number of clients and hangs
with over 60 clients without the patch; exact time was not re-measured
with patch)

Updated sanityn.sh 43a 45a to avoid race conditions.

Add sanityn.sh test_43j to verify above scenario.

Test-Parameters: envdefinitions=SLOW=yes testlist=replay-vbr,replay-vbr
Change-Id: I37fc9c8ffc7ab334c0645042beda5bef01284564
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/30880
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>