namei.c: switch user pathname imports to CLASS(filename{,_flags})

namei.c: switch user pathname imports to CLASS(filename{,_flags})

filename_flags is used by user_path_at().  I suspect that mixing
LOOKUP_EMPTY with real lookup flags had been a mistake all along; the
former belongs to pathname import, the latter - to pathwalk.  Right now
none of the remaining in-tree callers of user_path_at() are getting
LOOKUP_EMPTY in flags, so user_path_at() could probably be switched
to CLASS(filename)...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>