iov_iter: set nr_segs = 1 for ITER_UBUF

JIRA: https://issues.redhat.com/browse/RHEL-87739

commit cd0bd57
Author: Jens Axboe <axboe@kernel.dk>
Date:   Tue Mar 28 14:29:03 2023 -0600

    iov_iter: set nr_segs = 1 for ITER_UBUF

    To avoid needing to check if a given user backed iov_iter is of type
    ITER_IOVEC or ITER_UBUF, set the number of segments for the ITER_UBUF
    case to 1 as we're carrying a single segment.

    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

include/linux/uio.h

@@ -396,7 +396,8 @@ static inline void iov_iter_ubuf(struct iov_iter *i, unsigned int direction,
 		.user_backed = true,
 		.data_source = direction,
 		.ubuf = buf,
-		.count = count
+		.count = count,
+		.nr_segs = 1
 	};
 }
 /* Flags for iov_iter_get/extract_pages*() */