erofs: relaxed temporary buffers allocation on readahead

Even with inplace decompression, sometimes very few temporary buffers
may be still needed for a single decompression shot (e.g. 16 pages for
64k sliding window or 4 pages for 16k sliding window).  In low-memory
scenarios, it would be better to try to allocate with GFP_NOWAIT on
readahead first.  That can help reduce the time spent on page allocation
under durative memory pressure.

Here are detailed performance numbers under multi-app launch benchmark
workload [1] on ARM64 Android devices (8-core CPU and 8GB of memory)
running a 5.15 LTS kernel with EROFS of 4k pclusters:

+----------------------------------------------+
|      LZ4       | vanilla | patched |  diff   |
|----------------+---------+---------+---------|
|  Average (ms)  |  3364   |  2684   | -20.21% | [64k sliding window]
|----------------+---------+---------+---------|
|  Average (ms)  |  2079   |  1610   | -22.56% | [16k sliding window]
+----------------------------------------------+

The total size of system images for 4k pclusters is almost unchanged:
(64k sliding window)  9,117,044 KB
(16k sliding window)  9,113,096 KB

Therefore, in addition to switch the sliding window from 64k to 16k,
after applying this patch, it can eventually save 52.14% (3364 -> 1610)
on average with no memory reservation.  That is particularly useful for
embedded devices with limited resources.

[1] https://lore.kernel.org/r/20240109074143.4138783-1-guochunhai@vivo.com



Suggested-by: Gao Xiang <xiang@kernel.org>
Signed-off-by: Chunhai Guo <guochunhai@vivo.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Link: https://lore.kernel.org/r/20240126140142.201718-1-hsiangkao@linux.alibaba.com