powerpc/code-patching: Reorganise do_patch_instruction() to ease error handling

Split do_patch_instruction() in two functions, the caller doing the
spin locking and the callee doing everything else.

And remove a few unnecessary initialisations and intermediate
variables.

This allows the callee to return from anywhere in the function.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/dbc85980a0d2a935731b272e8907e8bb1d8fc8c5.1638446239.git.christophe.leroy@csgroup.eu