LU-5417 osp: fix comparison between signed and unsigned

LU-5417 osp: fix comparison between signed and unsigned

Make oxe_buflen, oxe_namelen and oxe_vallen unsigned in
struct osp_xattr_entry.

Signed-off-by: Dmitry Eremin <dmitry.eremin@intel.com>
Change-Id: I087772016b52d8f14d60c83dcf4a1923cca76df0
Reviewed-on: http://review.whamcloud.com/11315
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>