gfs2: Don't release and reacquire local statfs bh

Before this patch, several functions in gfs2 related to the updating
of the statfs file used a newly acquired/read buffer_head for the
local statfs file. This is completely unnecessary, because other nodes
should never update it. Recreating the buffer is a waste of time.

This patch allows gfs2 to read in the local statefs buffer_head at
mount time and keep it around until unmount time.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>