sparc64: fix misuses of access_process_vm() in genregs32_[sg]et()

We do need access_process_vm() to access the target's reg_window.
However, access to caller's memory (storing the result in
genregs32_get(), fetching the new values in case of genregs32_set())
should be done by normal uaccess primitives.

Fixes: ad4f9576 ([SPARC64]: Fix user accesses in regset code.)
Cc: stable@kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>