Ext4: fix handling of files with sparse blocks before extent's index.

Ext4: fix handling of files with sparse blocks before extent's index.

This is ongoing work from Damjan Jovanovic to improve ext4 read support
with sparse files:

Keep track of the first and last block in each extent as it descends down
the extent tree, thus being able to work out that some blocks are sparse
earlier. This solves an issue on r293680.

In ext4_bmapext() start supporting the runb parameter, which appears to be
the number of adjacent blocks prior to the block being converted in the
same way that runp is the number of blocks after, speding up random access
to mmaped files.

PR:	206652