LU-4219 mdd: limit os_namelen to the max of NAME_MAX

LU-4219 mdd: limit os_namelen to the max of NAME_MAX

While running pathconf(".", _PC_NAME_MAX) on ldiskfs-based
Lustre filesystem, it returns 255 (NAME_MAX). However, on
zfs-based Lustre filesystem, it returns 256 (MAXNAMELEN).
Since Lustre can not handle the name length large than
NAME_MAX, this patch limits the value of os_namelen in
mdd_statfs() to the max of NAME_MAX.

Test-Parameters: envdefinitions=SLOW=yes \
mdtfilesystemtype=zfs mdsfilesystemtype=zfs ostfilesystemtype=zfs \
ostcount=2 testlist=posix

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I13b5398eff5b15c74b0e08e2a3394b79a3ff6a44
Reviewed-on: http://review.whamcloud.com/8217
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>