f2fs: fix long latency due to discard during umount

F2FS already has a default timeout of 5 secs for discards that
can be issued during umount, but it can take more than the 5 sec
timeout if the underlying UFS device queue is already full and there
are no more available free tags to be used. Fix this by submitting a
small batch of discard requests so that it won't cause the device
queue to be full at any time and thus doesn't incur its wait time
in the umount context.

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>