arm64: hw_breakpoint: Handle inexact watchpoint addresses

Arm64 hardware does not always report a watchpoint hit address that
matches one of the watchpoints set. It can also report an address
"near" the watchpoint if a single instruction access both watched and
unwatched addresses. There is no straight-forward way, short of
disassembling the offending instruction, to map that address back to
the watchpoint.

Previously, when the hardware reported a watchpoint hit on an address
that did not match our watchpoint (this happens in case of instructions
which access large chunks of memory such as "stp") the process would
enter a loop where we would be continually resuming it (because we did
not recognise that watchpoint hit) and it would keep hitting the
watchpoint again and again. The tracing process would never get
notified of the watchpoint hit.

This commit fixes the problem by looking at the watchpoints near the
address reported by the hardware. If the address does not exactly match
one of the watchpoints we have set, it attributes the hit to the
nearest watchpoint we have.  This heuristic is a bit dodgy, but I don't
think we can do much more, given the hardware limitations.

Signed-off-by: Pavel Labath <labath@google.com>
[panand: reworked to rebase on his patches]
Signed-off-by: Pratyush Anand <panand@redhat.com>
[will: use __ffs instead of ffs - 1]
Signed-off-by: Will Deacon <will.deacon@arm.com>