nsfs: generalize ns_get_path() for path resolution with a task

ns_get_path() takes struct task_struct and proc_ns_ops as its
parameters.  For path resolution directly from a namespace,
e.g. based on a networking device's net name space, we need
more flexibility.  Add a ns_get_path_cb() helper which will
allow callers to use any method of obtaining the name space
reference.  Convert ns_get_path() to use ns_get_path_cb().

Following patches will bring a networking user.

CC: Eric W. Biederman <ebiederm@xmission.com>
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>