powerpc/ptrace: Create ptrace_get_fpr() and ptrace_put_fpr()

On the same model as ptrace_get_reg() and ptrace_put_reg(),
create ptrace_get_fpr() and ptrace_put_fpr() to get/set
the floating points registers.

We move the boundary checkings in them.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/24a1baedea7f7ae7b6bf27be98bab6d01b5ca2c1.1597770847.git.christophe.leroy@csgroup.eu