first set of iomap fix

[Christoph Hellwig]

This contains various fixes for the iomap path for 4.8, but it skips
the changes to the delalloc path (which aren't really about the iomap
path itself anyway).

Two fixes to make the iomap buffered write path catch up with changes
to the filemap one.  Then I've cherry picked the FIEMAP_FLAG_SYNC patch
from Dave, and added my version of re-implementing FIEMAP_FLAG_XATTR
as well.