BACKPORT: f2fs: introduce memory mode

Introduce memory mode to supports "normal" and "low" memory modes.
"low" mode is to support low memory devices. Because of the nature of
low memory devices, in this mode, f2fs will try to save memory sometimes
by sacrificing performance. "normal" mode is the default mode and same
as before.

Bug: 232003054
Bug: 267580491
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit 60f60d1f)
Change-Id: I7cb719b18f0002d7af47f7a18e8ec2f4c534bdd9