MAINTAINERS: add test_vmalloc.c to VMALLOC section

MAINTAINERS: add test_vmalloc.c to VMALLOC section

A vmalloc subsystem includes "lib/test_vmalloc.c" test suite.  Add an "F:"
entry under VMALLOC section to track this file as part of the subsystem.

Link: https://lkml.kernel.org/r/20250417161216.88318-3-urezki@gmail.com
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Christop Hellwig <hch@infradead.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>