KVM: Bound the number of dirty ring entries in a single reset at INT_MAX

KVM: Bound the number of dirty ring entries in a single reset at INT_MAX

Cap the number of ring entries that are reset in a single ioctl to INT_MAX
to ensure userspace isn't confused by a wrap into negative space, and so
that, in a truly pathological scenario, KVM doesn't miss a TLB flush due
to the count wrapping to zero.  While the size of the ring is fixed at
0x10000 entries and KVM (currently) supports at most 4096, userspace is
allowed to harvest entries from the ring while the reset is in-progress,
i.e. it's possible for the ring to always have harvested entries.

Opportunistically return an actual error code from the helper so that a
future fix to handle pending signals can gracefully return -EINTR.  Drop
the function comment now that the return code is a stanard 0/-errno (and
because a future commit will add a proper lockdep assertion).

Opportunistically drop a similarly stale comment for kvm_dirty_ring_push().

Cc: Peter Xu <peterx@redhat.com>
Cc: Yan Zhao <yan.y.zhao@intel.com>
Cc: Maxim Levitsky <mlevitsk@redhat.com>
Cc: Binbin Wu <binbin.wu@linux.intel.com>
Fixes: fb04a1eddb1a ("KVM: X86: Implement ring-based dirty memory tracking")
Reviewed-by: James Houghton <jthoughton@google.com>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20250516213540.2546077-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>