arm64: entry: Free up another register on kpti's tramp_exit path

commit 03aff3a77a58b5b52a77e00537a42090ad57b80b upstream.

Kpti stashes x30 in far_el1 while it uses x30 for all its work.

Making the vectors a per cpu data structure will require a second
register.

Allow tramp_exit two registers before it unmaps the kernel by
leaving x30 on the stack and stashing x29 in far_el1.

Change-Id: I324402ec90a8401780ac88d6ed91eaa50fb1911a
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Git-commit: 51acb811
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Kishor Krishna Bhat <quic_kishkris@quicinc.com>