cifs: Fix oops due to uninitialised variable

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

cifs: Fix oops due to uninitialised variable

Fix smb3_init_transform_rq() to initialise buffer to NULL before calling netfs_alloc_folioq_buffer() as netfs assumes it can append to the buffer it is given. Setting it to NULL means it should start a fresh buffer, but the value is currently undefined.