LU-14504 lod: lod_xattr_del() check obj existence

LU-14504 lod: lod_xattr_del() check obj existence

lod_declare_xattr_del() skips object if it doesn't exist, but
lod_xattr_del() doesn't, which may trigger assertion in
osp_xattr_del() if a stripe doesn't exist.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I00723d3b0243efd1357107c59dd86967e076e2af
Reviewed-on: https://review.whamcloud.com/41976
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>