x86/mm: fix memory hotplug error cleanup

x86/mm: fix memory hotplug error cleanup

During destroying the m2p mapping, the loop variable was always incremented
by one, as the current version used a compare operator on the left hand side,
which always evaluated to true, i.e.

i += 1UL < (L2_PAGETABLE_SHIFT - 2)

The fix increments the value of the variable by the actual page size by
using the shift operator instead.

Signed-off-by: Norbert Manthey <nmanthey@amazon.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>