x86/memshr: properly check grant references

x86/memshr: properly check grant references

They need to be range checked against the current table limit in any
event.

Reported-by: Huawei PSIRT <psirt@huawei.com>
Move the code to where it belongs, eliminating a number of duplicate
definitions. Add locking. Produce proper error codes, and consume them
instead of making one up. Check grant type. Convert parameter types at
once.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>