LU-15546 mdt: mdt_reint_open lookup before locking

LU-15546 mdt: mdt_reint_open lookup before locking

This patch is an optimization of 33dc40d ("LU-10262 mdt:
mdt_reint_open: check EEXIST without lock").

The current behavior is to take a LCK_PR on parent to verify if the
file exist and then take a LCK_PW to create the file.

Here we do a lookup to determine the mode before tacking a lock.
This avoid to re-lock each time for create cases.

Most of the time we have:
1. lookup the child in parent directory
2. take the parent lock: file_exist ? LCK_PR : LCK_PW
3. re-lookup the child

In a race senario (create/unlink) we have:
1. lookup child in parent directory -> file exists
2. take a LCK_PR on the parent
3. re-lookup the child -> file doesn't exist
2. take a LCK_PW on the parent
4. re-lookup the child

This patch fix the "SKIP" condition for sanityn 41i/43k/45j and clear
the LRU locks cache for sanityn 43k/45j.

Fixes: 33dc40d ("LU-10262 mdt: mdt_reint_open: check EEXIST without lock")
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Change-Id: I121abd4babfb516d7a64682b054a6443d38590ef
Reviewed-on: https://review.whamcloud.com/46679
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>