Linux 5.16 compat: restore FSR and FSAVE

Commit 3b52ccd7d introduced a flaw where FSR and FSAVE are not restored
when using a Linux 5.16 kernel. These instructions are only used when
XSAVE is not supported by the processor meaning only some systems will
encounter this issue.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Attila Fülöp <attila@fueloep.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13210
Closes #13236