gnttab: fix/adjust gnttab_transfer()

gnttab: fix/adjust gnttab_transfer()

- don't update shared entry's frame number for translated domains (as
  MFNs shouldn't be exposed to such guests)
- for v1 grant table format, force copying of the page also when the
  intended MFN doesn't fit in 32 bits (and the domain isn't translated)
- fix an apparent off-by-one error (it's unclear to me why commit
  5cc77f9098 ("32-on-64: Fix domain address-size clamping, implement")
  uses BITS_PER_LONG-1 here, while using BITS_PER_LONG in the two other
  invocations of domain_clamp_alloc_bitsize())
- adjust comments accompanying the shared entry's frame field

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>