x86/p2m: don't ignore p2m_remove_page()'s return value

x86/p2m: don't ignore p2m_remove_page()'s return value

It's not very nice to return from guest_physmap_add_entry() after
perhaps already having made some changes to the P2M, but this is pre-
existing practice in the function, and imo better than ignoring errors.

Take the liberty and replace an mfn_add() instance with a local variable
already holding the result (as proven by the check immediately ahead).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>