GFS2: fs/gfs2/file.c: kernel-doc warning fixes

Related function is not gfs2_set_flags but do_gfs2_set_flags

Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>