LU-10262 mdt: mdt_reint_open: check EEXIST without lock

LU-10262 mdt: mdt_reint_open: check EEXIST without lock

Many applications blindly open files with O_CREAT, and the mds gets a
write lock to the parent directory for these even if the file already
exists.
Checking for file existence first lets us take a PR lock if file
already existed even if O_CREAT was specified.

This opens up multiple races between the first lookup and the actual
locking, in each of them drop the resources we aquired and retry from
scratch to keep things as far from complicated as possible, with mixed
success.

Update (eaujames):
 - rebase the patch
 - update tests

Performance tests results:

The array below presents the average "open" syscall latency for 20
files in a single directory accessed by 400 different clients.
 _______________________________________________________________
| Test cases        | without patch | with patch | %improvement |
|___________________|_______________|____________|______________|
| readonly          | 0.960s        | 0.973s     | -1.40%       |
|___________________|_______________|____________|______________|
| readonly cached   | 0.372s        | 0.372s     | +0.01%       |
|___________________|_______________|____________|______________|
| O_CREAT+precreate | 1.645s        | 0.968s     | +41.13%      |
|___________________|_______________|____________|______________|
| O_CREAT cached    | 0.632s        | 0.623s     | +1.34%       |
|___________________|_______________|____________|______________|
| O_CREAT           | 1.261s        | 1.093s     | +13.32%      |
|___________________|_______________|____________|______________|
(for more detail, see the ticket comments section)

This patch optimizes concurent opens with O_CREAT flag when dentry are
not cached by clients.

Change-Id: I247b579d14d20036f89033c99ece457d70ba19e7
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Reviewed-on: https://review.whamcloud.com/33098
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>