mm: use vma_start_write_killable() in dup_mmap()

mm: use vma_start_write_killable() in dup_mmap()

Allow waiting for the VMA write lock to be interrupted by fatal signals.
The explicit check for fatal_signal_pending() can be removed as it is
checked during vma_start_write_killable().  Improves the latency of
killing the task as we do not wait for the reader to finish before
checking for signals.

Link: https://lkml.kernel.org/r/20251110203204.1454057-3-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Chris Li <chriscli@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>