LU-14405 mdt: read LMV with mdt_stripe_get()

LU-14405 mdt: read LMV with mdt_stripe_get()

mdt_path_current() reads LMV into mdt_thread_info.mti_xattr_buf,
whose size is static, and will return -ERANGE if LMV contains too
many stripes, instead it should call mdt_stripe_get(), the latter
will allocate dynamic memory for LMV.

Lustre-change: https://review.whamcloud.com/41452
Lustre-commit: 9dbfa36d3dd2434cfcffa13f76beb89fa3516586

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I1ed78f7a7f951fa5984e604a8773143a70b419e7
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/41966
Tested-by: jenkins <devops@whamcloud.com>