f2fs: fix to freeze GC and discard threads quickly

Suspend can fail if kernel threads do not freeze for a while.
f2fs_gc and f2fs_discard threads can perform long-running operations
that prevent them from reaching a freeze point in a timely manner.

This patch adds explicit freezing checks in the following locations:
1. f2fs_gc: Added a check at the 'retry' label to exit the loop quickly
if freezing is requested, especially during heavy GC rounds.
2. __issue_discard_cmd: Added a 'suspended' flag to break both inner and
outer loops during discard command issuance if freezing is detected
after at least one command has been issued.
3. __issue_discard_cmd_orderly: Added a similar check for orderly discard
to ensure responsiveness.

These checks ensure that the threads release locks safely and enter the
frozen state.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>