Pull in the gfs2 iomap-write changes: Tweak the existing code to
properly support iomap write and eliminate an unnecessary special case
in gfs2_block_map.  Implement iomap write support for buffered and
direct I/O.  Simplify some of the existing code and eliminate code that
is no longer used:

  gfs2: Remove gfs2_write_{begin,end}
  gfs2: iomap direct I/O support
  gfs2: gfs2_extent_length cleanup
  gfs2: iomap buffered write support
  gfs2: Further iomap cleanups

This is based on the following changes on the xfs 'iomap-4.19-merge'
branch:

  iomap: add private pointer to struct iomap
  iomap: add a page_done callback
  iomap: generic inline data handling
  iomap: complete partial direct I/O writes synchronously
  iomap: mark newly allocated buffer heads as new
  fs: factor out a __generic_write_end helper
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>