The expansion code for the directory exhash table currently works like this:

1. vmalloc a chunk of memory
2. read the current hash table into it
3. write out a new version doubling the size

It involved being able to allocate a lot of memory and potentially also
increasing latency by doing all the reading and then all the writing. A better
algorithm looks like this:

1. Expand the directory's data blocks to the newly required size allocating
blocks as required
2. Use two "pointers", one starts at the new end of the file, the other at the
original end of the file.
3. Copy the data from original to new moving towards the start of the file, the
"new" pointer will catch up with the "original" pointer only on the final copy
operation.
4. Update the i_size to indicate that the operation is complete

Potantially we might be able to increase the maximum size of the hash table
since the limiting factor appeared to be set only be the maximum size of memory
that it was reasonable to allocate using vmalloc. If this is done then we need
to check that it will remain backward compatible, but it would seem reasonable
that this should be the case.