LU-12911 llite: Don't access lov_md fields before size check

When 'struct lov_user_md' is passed in via setxattr, it comes with
a size.  If thatt size is too small, some function that check exactly
what version is present might access beyond the end of allocation
memory, which can have undesirable effects, such as triggering
a KASAN warning (and possibly worse).

So check that the size is sane before looking inside the structure
at all.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ib3f071a3ff77a039fdfa38c903d87999108b3322
Reviewed-on: https://review.whamcloud.com/36589
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>