Another trivial sem2mutex conversion.

Side note:  nodemgr_serialize's purpose, when introduced in linux1394's
revision 529 in July 2002, was to protect several data structures which
are now largely handled by or together with Linux' driver core and are
now protected by the LDM's own mechanisms.  It may very well be possible
to remove this mutex now.  But fully parallelized node scanning is on
our long-term TODO list anyway; the mutex will certainly go away then.
Signed-off-by: NStefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: NBen Collins <bcollins@ubuntu.com>