f2fs: return true if all cmd were issued or no cmd need to be issued for f2fs_issue_discard_timeout()

f2fs_issue_discard_timeout() returns whether discard cmds are dropped,
which does not match the meaning of the function. Let's change it to
return whether all discard cmd are issued.

After commit 4d674904 ("f2fs: Don't create discard thread when
device doesn't support realtime discard"), f2fs_issue_discard_timeout()
is alse called by f2fs_remount(). Since the comments of
f2fs_issue_discard_timeout() doesn't make much sense, let's update it.
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>