maple_tree: add mas_is_active() to detect in-tree walks

Patch series "maple_tree: Fix mas_prev() state regression".

Pedro Falcato retported an mprotect regression [1] which was bisected back
to the iterator changes for maple tree.  Root cause analysis showed the
mas_prev() running off the end of the VMA space (previous from 0) followed
by mas_find(), would skip the first value.

This patchset introduces maple state underflow/overflow so the sequence of
calls on the maple state will return what the user expects.

Users who encounter this bug may see mprotect(), userfaultfd_register(),
and mlock() fail on VMAs mapped with address 0.


This patch (of 2):

Instead of constantly checking each possibility of the maple state,
create a fast path that will skip over checking unlikely states.

Link: https://lkml.kernel.org/r/20230921181236.509072-1-Liam.Howlett@oracle.com
Link: https://lkml.kernel.org/r/20230921181236.509072-2-Liam.Howlett@oracle.com


Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>