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.

Change-Id: Ib9669453795601e9e602c204aad31a5cb70688a4
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Git-commit: 141af6ba5216d4d49de683582b600d5c9b51792c
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/


[stummala@codeaurora.org: Fix trivial merge conflicts]
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>