arm64: fix possible spectre-v1 in ptrace_hbp_get_event()

arm64: fix possible spectre-v1 in ptrace_hbp_get_event()

BugLink: http://bugs.launchpad.net/bugs/1794889
[ Upstream commit 19791a7ca674fb3009bb068260e852a2f05b605c ]

It's possible for userspace to control idx. Sanitize idx when using it
as an array index.

Found by smatch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>