maple tree: use goto label to simplify code

Use the underflow goto label to set the status to ma_underflow and return
NULL, as is being done elsewhere.

[akpm@linux-foundation.org: add newline, per Liam (and remove one, per akpm)]
Link: https://lkml.kernel.org/r/20250624080748.4855-1-dev.jain@arm.com


Signed-off-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>