x86/mm: Use a more descriptive name for pagetable mfns

x86/mm: Use a more descriptive name for pagetable mfns

In many places, a PTE being modified is accompanied by the pagetable
mfn which contains the PTE (primarily in order to be able to maintain
linear mapping counts).  In many cases, this mfn is stored in the
non-descript variable (or argement) "pfn".

Replace these names with lNmfn, to indicate that 1) this is a
pagetable mfn, and 2) that it is the same level as the PTE in
question.  This should be enough to remind readers that it's the mfn
containing the PTE.

No functional change.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>