x86/mm: drop redundant domain parameter from get_page_from_gfn_p2m()

x86/mm: drop redundant domain parameter from get_page_from_gfn_p2m()

It can always be read from the passed p2m. Take the opportunity and
also rename the function, making the "p2m" suffix a prefix, to match
other p2m functions, and convert the "gfn" parameter's type.

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