CVE-2023-52598: s390/ptrace: handle setting of fpc register correctly

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

s390/ptrace: handle setting of fpc register correctly

If the content of the floating point control (fpc) register of a traced process is modified with the ptrace interface the new value is tested for validity by temporarily loading it into the fpc register.

This may lead to corruption of the fpc register of the tracing process: if an interrupt happens while the value is temporarily loaded into the fpc register, and within interrupt context floating point or vector registers are used, the current fp/vx registers are saved with savefpuregs() assuming they belong to user space and will be loaded into fp/vx registers when returning to user space.

testfpctl() restores the original user space fpc register value, however it will be discarded, when returning to user space.

In result the tracer will incorrectly continue to run with the value that was supposed to be used for the traced process.

Fix this by saving fpu register contents with savefpuregs() before using testfpctl().