mm: nommu: correct the range of mmap_sem_read_lock in task_mem()

During the seq_printf,the mmap_sem_read_lock protection is not
required.

Link: https://lkml.kernel.org/r/20230622040152.1173-1-lipeifeng@oppo.com


Signed-off-by: lipeifeng <lipeifeng@oppo.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Liam R. Howlett <Liam.Howlett@oracle.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>