LU-12602 mdt: more EA size check in mdt_getxattr_pack_reply()

While the RMF_EAVALS field size can be arbitrary length,
the RMF_EAVALS_LENS field definition specifies
the RMF_F_STRUCT_ARRAY flag, so the passed size must be a multiple
of sizeof(__u32) or the internal LBUG() will trigger.

Signed-off-by: Emoly Liu <emoly@whamcloud.com>
Change-Id: I767e1b1496298e9a66274fc324f9c34daaed4a09
Reviewed-on: https://review.whamcloud.com/36103
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>