The pdflush thread is long gone, so this patch removes references to pdflush
from gfs comments.

Cc: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: NArtem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>