ext4: add new helper interface ext4_try_to_trim_range()

[ Upstream commit 6920b3913235f517728bb69abe9b39047a987113 ]

There is no functional change in this patch but just split the
codes, which serachs free block and does trim, into a new function
ext4_try_to_trim_range. This is preparing for the following async
backgroup discard.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Wang Jianchao <wangjianchao@kuaishou.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20210724074124.25731-3-jianchao.wan9@gmail.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Stable-dep-of: 45e4ab320c9b ("ext4: move setting of trimmed bit into ext4_try_to_trim_range()")
Signed-off-by: Sasha Levin <sashal@kernel.org>