[PATCH] staging: vme_user: validate slave window size against buffer size

[木口璃音]

This patch addresses the OOB read/write reported earlier
in the security@xxxxxxxxxx thread (now handled publicly per
Greg's and Willy's guidance).

Tested on Linux 7.1.0-rc2 with KASAN; all three reproducers
fail with -EINVAL after applying this patch and produce no
KASAN splat.