lib: Generalize parse_mapping()

lib: Generalize parse_mapping()

The function parse_mapping() assumes the key is a number, with a single
configurable exception, which is using "all" to mean "all possible keys".
If a caller wishes to use symbolic names instead of numbers, they cannot
reuse this function.

To facilitate reuse in these situations, convert parse_mapping() into a
helper, parse_mapping_gen(), which instead of an allow-all boolean takes a
generic key-parsing callback. Rewrite parse_mapping() in terms of this
newly-added helper and add a pair of key parsers, one for just numbers,
another for numbers and the keyword "all". Publish the latter as well.

Signed-off-by: Petr Machata <me@pmachata.org>
Signed-off-by: David Ahern <dsahern@kernel.org>