ext4: don't lock the next page in write_cache_pages if not needed

If we have accumulated a contiguous region of memory to be written
out, and the next page can added to this region, don't bother locking
(and then unlocking the page) before writing out the memory.  In the
unlikely event that the next page was being written back by some other
CPU, we can also skip waiting that page to finish writeback.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>