util: increase column's nullBitmap's capacity (#12470)

[H-ZeX]

cherry-pick #12470 to release-3.1

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Signed-off-by: H-ZeX hzx20112012@gmail.com

What problem does this PR solve?

nullBitmap's cap

The origin nullBitmap's capacity is a little small.
When the cap param is (x<<3)+y(0<y<8), the nullBitMap's len is only x, so the col that it can represent is (x<<3), which is small that the column number (x<<3)+y, then this slice need to be expanded.
For example, if there is 8+7=15 col, we need 2 bytes nullBitmap to represent it. But, 15>>3 only get 1, so this slice need to be expanded to 2 bytes.
If the slice expands, the memory usage increased is not little, so I think adding one byte to nullBitmap is better and necessary.
In util/chunk/column.go, there is many patterns like (x+7)>>3 such as

func (c *Column) appendMultiSameNullBitmap(notNull bool, num int) {
	numNewBytes := ((c.length + num + 7) >> 3) - len(c.nullBitmap)

sizeNulls := (n + 7) >> 3

What is changed and how it works?

• x>>3 -> ((x+7)>>3)

Check List

Tests

existing unit test

Side effects

No.

Related changes

No.

Release note

TODO

[zimulala]

LGTM

[lonng]

LGTM

[sre-bot]

/run-all-tests