x86/mem-paging: fold p2m_mem_paging_prep()'s main if()-s

x86/mem-paging: fold p2m_mem_paging_prep()'s main if()-s

The condition of the second can be true only if the condition of the
first was met; the second half of the condition of the second then also
is redundant with an earlier check. Combine them, drop a pointless
local variable, and take the liberty to drop the affected gdprintk()
altogether, as we don't normally log anything on -EFAULT paths.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>