get_user_instr() lacks sparse detection for the __user tag.

This is because __gui_ptr is assigned with a cast.

Fix that by adding a __chk_user_ptr()
Signed-off-by: NChristophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0320e5b41a794fd456ab8c5993bbfadcf9e1d8b4.1621516826.git.christophe.leroy@csgroup.eu