CVE-2026-72366: netfs: Fix netfs_create_write_req() to handle async cache object creation

In the Linux kernel, the following vulnerability has been resolved:

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

netfscreatewritereq() 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 fscachebeginwriteoperation() to be skipped.

Fix this by removing the checks on the cookie and delegating this to fscachebeginwriteoperation().