LU-16464 osp: fix off-by-one errors in oxe_can_hold()

LU-16464 osp: fix off-by-one errors in oxe_can_hold()

There are a couple of off-by-one errors when calculating the required
buffer size in oxe_can_hold(), which can cause the xattr entry to be
reallocated unnecessarily.

HPE-bug-id: LUS-11423
Fixes: a1c5adf7f466 ("LU-14607 osp: separate buffer for large XATTR")
Change-Id: I486963066d7f8783ad64f1ea110fb73db0a8274b
Signed-off-by: Nikitas Angelinas <nikitas.angelinas@hpe.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49617
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>