quota: don't set grace time when user isn't above softlimit

do_set_dqblk() allowed SETDQBLK quotactl to set user's grace time even if
user was not above his softlimit.  This does not make much sence and by
coincidence causes quota code to omit softlimit warning when user really
exceeds softlimit.  This patch makes do_set_dqblk() reset user's grace
time if he has not exceeded softlimit.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>