x86/np2m: simplify nestedhvm_hap_nested_page_fault()

x86/np2m: simplify nestedhvm_hap_nested_page_fault()

There is a possibility for nested_p2m to became stale between
nestedhvm_hap_nested_page_fault() and nestedhap_fix_p2m().  At the moment
this is handled by detecting such a race inside nestedhap_fix_p2m() and
special-casing it.

Instead, introduce p2m_get_nestedp2m_locked(), which will returned a
still-locked p2m.  This allows us to call nestedhap_fix_p2m() with the
lock held and remove the code detecting the special-case.

Signed-off-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jun Nakajima <jun.nakajima@intel.com>