bcachefs: Add error handling for zlib_deflateInit2()

In attempt_compress(), the return value of zlib_deflateInit2() needs to be
checked. A proper implementation can be found in  pstore_compress().

Add an error check and return 0 immediately if the initialzation fails.

Fixes: 986e9842 ("bcachefs: Compression levels")
Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>