mm/mmap_lock: add vma_is_attached() helper

This makes it easy to explicitly check for VMA detachment, which is useful
for things like asserts.

Note that we intentionally do not allow this function to be available
should CONFIG_PER_VMA_LOCK be set - this is because vma_assert_attached()
and vma_assert_detached() are no-ops if !CONFIG_PER_VMA_LOCK, so there is
no correct state for vma_is_attached() to be in if this configuration
option is not specified.

Therefore users elsewhere must invoke this function only after checking
for CONFIG_PER_VMA_LOCK.

We rework the assert functions to utilise this.

Link: https://lkml.kernel.org/r/0172d3bf527ca54ba27d8bce8f8476095b241ac7.1768746221.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Chris Li <chriscli@google.com>
Cc: David Hildenbrand <david@kernel.org>
Cc: Harry Yoo <harry.yoo@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Rik van Riel <riel@surriel.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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