mm: Fix comment before truncate_setsize()

XFS doesn't always hold i_mutex when calling truncate_setsize() and it
uses a different lock to serialize truncates and writes. So fix the
comment before truncate_setsize().

Reported-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dave Chinner <david@fromorbit.com>