mem_sharing: fix race condition of nominate and unshare

mem_sharing: fix race condition of nominate and unshare

(1) When updating/checking p2m type for mem_sharing, we must hold shr_lock
(2) For nominate operation, if the page is already nominated, return the
    handle from page_info->shr_handle
(3) For unshare operation, it is possible that multiple users unshare a
    page via hvm_hap_nested_page_fault() at the same time. If the page
    is already un-shared by someone else, simply return success.

Signed-off-by: Jui-Hao Chiang <juihaochiang@gmail.com>
Signed-off-by: Han-Lin Li <Han-Lin.Li@itri.org.tw>
Acked-by: Tim Deegan <Tim.Deegan@citrix.com>