erofs: avoid allocating DEFLATE streams before mounting

Currently, each DEFLATE stream takes one 32 KiB permanent internal
window buffer even if there is no running instance which uses DEFLATE
algorithm.

It's unexpected and wasteful on embedded devices with limited resources
and servers with hundreds of CPU cores if DEFLATE is enabled but unused.

Fixes: ffa09b3b ("erofs: DEFLATE compression support")
Cc: <stable@vger.kernel.org> # 6.6+
Reviewed-by: Sandeep Dhavale <dhavale@google.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240520090106.2898681-1-hsiangkao@linux.alibaba.com