riscv: fp: add missing __user pointer annotations

The __user annotations were removed from the {save,restore}_fp_state()
function signatures by commit 007f5c35

 ("Refactor FPU code in
signal setup/return procedures"), but should be present, and sparse
warns when they are not applied.  Add them back in.

This change should have no functional impact.

Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>
Fixes: 007f5c35

 ("Refactor FPU code in signal setup/return procedures")
Cc: Alan Kao <alankao@andestech.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>