netfs: Fix netfs_create_write_req() to handle async cache object creation

In the Linux kernel, the following vulnerability has been resolved: netfs: Fix netfs_create_write_req() to handle async cache object creation netfs_create_write_req() will skip caching if the fscache cookie is disabled, but this is a problem because async cache object creation might not have got far enough yet that has been enabled - thereby causing the call to fscache_begin_write_operation() to be skipped. Fix this by removing the checks on the cookie and delegating this to fscache_begin_write_operation().