A patch from Hirokazu Takahashi to speed up the new sped-up writev
code.

Instead of running ->prepare_write/->commit_write for each individual
segment, we walk the segments between prepage and commit.  So
potentially much larger amounts of data are passed to commit_write(),
and prepare_write() is called much less often.

Added bonus: the segment walk happens inside the kmap_atomic(), so we
run kmap_atomic() once per page, not once per segment.

We've demonstrated a speedup of over 3x.  This is writing 1024-segment
iovecs where the individual segments have an average length of 24
bytes, which is a favourable case for this patch.