lib/idmapping.c: get_map_ranges(): Rename local variable

lib/idmapping.c: get_map_ranges(): Rename local variable

For a pointer iterator used often, a single-letter identifier is more
appropriate.  That reduces the length of lines considerably, avoiding
unnecessary line breaks.  And since we initialize it with

	m = mappings;

it's clear what it is.

Link: <https://github.com/shadow-maint/shadow/commit/ff2baed5dbf81e8967b805889f565fedb48600df#r136635300>
Cc: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>