disk bucket: get mut right on get_mut_cell_slice

[jeffwashington]

Problem

see #30711

get_mut_cell_slice() should have taken &mut self but took &self due to borrow checker difficulties.

Summary of Changes

correctly use &mut self

Fixes #

[brooksprumo]

How about this for the impl:

    pub fn get_mut_cell_slice<T: Sized>(&mut self, ix: u64, len: u64) -> &mut [T] {
        let start = self.get_start_offset_no_header(ix);
        let end = start + std::mem::size_of::<T>() * len as usize;
        //debug!("GET mut slice {} {}", start, end);
        let item_slice = &mut self.mmap[start..end];
        let item = item_slice.as_mut_ptr() as *mut T;
        unsafe { std::slice::from_raw_parts_mut(item, len as usize) }
    }

And then a subsequent PR can change get_mut() to call this fn with len = 1

[brooksprumo]

Will handle in a subsequent PR.

[brooksprumo]

#30985

[codecov]

Codecov Report

Merging #30972 (355f1b2) into master (2646fb7) will increase coverage by 0.0%.
The diff coverage is 67.8%.

@@           Coverage Diff           @@
##           master   #30972   +/-   ##
=======================================
  Coverage    81.5%    81.5%           
=======================================
  Files         728      728           
  Lines      205467   205454   -13     
=======================================
+ Hits       167532   167543   +11     
+ Misses      37935    37911   -24     

[brooksprumo]

Lgtm