hash-func: add generic hash_ops implementation for hashing paths

This is similar to string_hash_ops but operates one file system paths
specifically. It will ensure that "/foo//bar" and "///foo/bar" are
considered to be the same path for hashmap purposes.

This makes use of the existing path_compare() API, and adds a matching
hashing function for it.

Note that relative and absolute paths will hash to different values,
however whether the path is suffixed with a slash or not is not
detected. This matches the existing path_compare() behaviour, and
follows the logic that on Linux there can't be two different objects at
path /foo/bar and /foo/bar/ either.