[GFS2] sem -> mutex conversion in locking.c

Convert a semaphore to a mutex in locking.c and also tidy
up one or two loose ends.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

diff --cc fs/gfs2/locking.c
Simple merge

diff --cc fs/gfs2/log.c
Simple merge

diff --cc fs/gfs2/util.h
Simple merge