zoned xfs updates v2

Hi Carlos,

this series as a bunch of zones fixes, and a trivial code movement to
move the ioend code into it's own file.

Andrey just sent a basically identical version of the third patch, but
I'm including here as this is the series I tested.  Feel free to take his
version and take my indendent reimplementation as a reviewed-by.

Changes since v1:
 - fix a commit message typo
 - clarify the reuse of an old zone in set_zoned_cached by better code
   structure and comments

Diffstat:
 Makefile         |    1 
 xfs_aops.c       |  188 -------------------------------------------------------
 xfs_aops.h       |    1 
 xfs_file.c       |    2 
 xfs_ioend.c      |  184 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 xfs_ioend.h      |   16 ++++
 xfs_iomap.c      |    7 --
 xfs_iomap.h      |   14 ++++
 xfs_zone_alloc.c |   66 +++++++++++++++----
 9 files changed, 273 insertions(+), 206 deletions(-)