lib: hex2bin converts ascii hexadecimal string to binary

Similar to the kgdb_hex2mem() code, hex2bin converts a string
to binary using the hex_to_bin() library call.

Changelog:
- Replace parameter names with src/dst (based on David Howell's comment)
- Add 'const' where needed (based on David Howell's comment)
- Replace int with size_t (based on David Howell's comment)

Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Acked-by: Serge E. Hallyn <serge@hallyn.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>