powerpc/ptrace: Move set_return_regs_changed() before regs_set_return_{msr/ip}

regs_set_return_msr() and regs_set_return_ip() have a copy
of the code of set_return_regs_changed().

Move up set_return_regs_changed() so it can be reused by
regs_set_return_{msr/ip}

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/49f4fb051a3e1cb69f7305d5b6768aec14727c32.1624619582.git.christophe.leroy@csgroup.eu