mm: (k)vrealloc: document concurrency restrictions

mm: (k)vrealloc: document concurrency restrictions

Document that concurrent calls to vrealloc() with itself or vfree() for
the same memory allocation are not permitted, since it leads to undefined
behavior.

Consequently, the same applies for kvrealloc(), hence also document it
for kvrealloc().

Link: https://lkml.kernel.org/r/20240725125442.4957-1-dakr@kernel.org
Suggested-by: Christoph Hellwig <hch@infradead.org>
Suggested-by: Uladzislau Rezki <urezki@gmail.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>