powerpc64/ftrace: fix clobbered r15 during livepatching

In the Linux kernel, the following vulnerability has been resolved:

powerpc64/ftrace: fix clobbered r15 during livepatching

While r15 is clobbered always with PPC_FTRACE_OUT_OF_LINE, it is

not restored in livepatch sequence leading to not so obvious fails

like below:

BUG: Unable to handle kernel data access on write at 0xc0000000000f9078

Faulting instruction address: 0xc0000000018ff958

Oops: Kernel access of bad area, sig: 11 [#1]

...

NIP: c0000000018ff958 LR: c0000000018ff930 CTR: c0000000009c0790

REGS: c00000005f2e7790 TRAP: 0300 Tainted: G K (6.14.0+)

MSR: 8000000000009033 <SF,EE,ME,IR,DR,RI,LE> CR: 2822880b XER: 20040000

CFAR: c0000000008addc0 DAR: c0000000000f9078 DSISR: 0a000000 IRQMASK: 1

GPR00: c0000000018f2584 c00000005f2e7a30 c00000000280a900 c000000017ffa488

GPR04: 0000000000000008 0000000000000000 c0000000018f24fc 000000000000000d

GPR08: fffffffffffe0000 000000000000000d 0000000000000000 0000000000008000

GPR12: c0000000009c0790 c000000017ffa480 c00000005f2e7c78 c0000000000f9070

GPR16: c00000005f2e7c90 0000000000000000 0000000000000000 0000000000000000

GPR20: 0000000000000000 c00000005f3efa80 c00000005f2e7c60 c00000005f2e7c88

GPR24: c00000005f2e7c60 0000000000000001 c0000000000f9078 0000000000000000

GPR28: 00007fff97960000 c000000017ffa480 0000000000000000 c0000000000f9078

...

Call Trace:

check_heap_object+0x34/0x390 (unreliable)

__mutex_unlock_slowpath.isra.0+0xe4/0x230

seq_read_iter+0x430/0xa90

proc_reg_read_iter+0xa4/0x200

vfs_read+0x41c/0x510

ksys_read+0xa4/0x190

system_call_exception+0x1d0/0x440

system_call_vectored_common+0x15c/0x2ec

Fix it by restoring r15 always.