mm/vmalloc.c: return explicit error value in alloc_vmap_area()

mm/vmalloc.c: return explicit error value in alloc_vmap_area()

In codes of alloc_vmap_area(), it returns the upper bound 'vend' to
indicate if the allocation is successful or failed.  That is not very
clear.

Here change to return explicit error values and check them to judge if
allocation is successful.

Link: https://lkml.kernel.org/r/20250418223653.243436-6-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Shivank Garg <shivankg@amd.com>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>