smb/client: avoid possible NULL dereference in cifs_free_subrequest()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: smb/client: avoid possible NULL dereference in cifs_free_subrequest() Clang static checker (scan-build) warning: cifsglob.h:line 890, column 3 Access to field 'ops' results in a dereference of a null pointer. Commit 519be989717c ("cifs: Add a tracepoint to track credits involved in R/W requests") adds a check for 'rdata->server', and let clang throw this warning about NULL dereference. When 'rdata->credits.value != 0 && rdata->server == NULL' happens, add_credits_and_wake_if() will call rdata->server->ops->add_credits(). This will cause NULL dereference problem. Add a check for 'rdata->server' to avoid NULL dereference.

References

• https://git.kernel.org/stable/c/fead60a6d5f84b472b928502a42c419253afe6c1
• https://git.kernel.org/stable/c/74c2ab6d653b4c2354df65a7f7f2df1925a40a51