staging: erofs: turn cache strategies into mount options

Kill all kconfig cache strategies and turn them into mount options
"cache_strategy={disable|readahead|readaround}".

As the first step, cached pages can still be usable after cache
is disabled by remounting, and these pages will be fallen out
over time, which can be refined in the later version if some
requirement is needed. Update related document as well.

Suggested-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Change-Id: I8753a190949a73b2864e5561a7b6d059da4e0684
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190731155752.210602-20-gaoxiang25@huawei.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>