LU-17144 mdt: set dmv by setxattr

LU-17144 mdt: set dmv by setxattr

Client side: convert setxattr("trusted.dmv") to "setdirstripe -D", as
will help restore directory default LMV from backup.

Server side: add a tunable to enable setxattr("trusted.dmv"), it can
be turned on by "lctl set_param -n mdt.*.enable_dmv_xattr=1". It's
off by default. Since empty buffer can be set by setxattr, add check
in server code to avoid crash.

Add sanity 413j.

Test-Parameter: serverversion=2.14 mdtcount=4 testlist=sanity env=ONLY=413j
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I27d784998a9c4a182b4fffb8b06c84e9d9190919