damon_test_three_regions_in_vmas() initializes a maple tree with
MM_MT_FLAGS.  The flags contains MT_FLAGS_LOCK_EXTERN, which means mt_lock
of the maple tree will not be used.  And therefore the maple tree
initialization code skips initialization of the mt_lock.  However,
__link_vmas(), which adds vmas for test to the maple tree, uses the
mt_lock.  In other words, the uninitialized spinlock is used.  The problem
becomes clear when spinlock debugging is turned on, since it reports
spinlock bad magic bug.

Fix the issue by excluding MT_FLAGS_LOCK_EXTERN from the maple tree
initialization flags.  Note that we don't use empty flags to make it
further similar to the usage of mm maple tree, and to be prepared for
possible future changes, as suggested by Liam.

Link: https://lkml.kernel.org/r/20240904172931.1284-1-sj@kernel.org
Fixes: d0cf3dd4 ("damon: convert __damon_va_three_regions to use the VMA iterator")
Signed-off-by: SeongJae Park <sj@kernel.org>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Closes: https://lore.kernel.org/1453b2b2-6119-4082-ad9e-f3c5239bf87e@roeck-us.netSuggested-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>