rx: remove trailing semicolons from FBSD mutex operations

rx: remove trailing semicolons from FBSD mutex operations

Since the first introduction of FreeBSD support, the macros
(MUTEX_ENTER, etc.) for kernel mutex operations have included
trailing semicolons, unique among all the platforms.

This did not cause problems until the recent work on rx event
handlers, which put a MUTEX_ENTER() in the body of an 'if' clause
with no brackets, and attempted to follow it with an 'else' clause.
This results in the following (rather obtuse) compiler error:

   /root/openafs/src/rx/rx.c:3666:5: error: expected expression
       else
       ^

Which is more visible in the preprocessed source, as

   if (condition)
       expression;;
   else
       other_expression;

is clearly invalid C.

To fix the FreeBSD kernel module build, remove the unneeded semicolons.

Change-Id: I191009ad412852dcc03cd71a0982fe41a953301d
Reviewed-on: https://gerrit.openafs.org/12853
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>