lustre: get rid of playing with ->fs

* removed several pieces of dead code in lustre_compat25.h
* don't open-code current_umask() (and BTW, 0755 & (S_IRWXUGO | S_ISVTX)
is better spelled as 0755)
* fix broken attempt to get the pathname by dentry - abusing d_path() for
that is simply wrong.

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