conver XFS perag lookup to xarrays

[Christoph Hellwig <hch@lst.de>]

Hi all,

for a project with the pending RT group code in XFS that reuses the basic
perag concepts I'd much prefer to use xarrays over the old radix tree for
nicer iteration semantics.

This series converts the perag code to xarrays to keep them in sync and
throws in the use of kfree_rcu_mightsleep in the same area.

To easily allow porting libxfs code to userspace this can't use xa_set
which requires stealing bits from the pointer, and as part of
investigating that I realized that the xa_set API is generally not so
nice for callers for which a pre-existing entry in the xarray is
always an error.  Thus the first patch adds a new xa_set wrapper that
treats an existing entry as an error and avoids the need to use of
xa_err.  It could also be used to clean up a significant number of
existing callers.

Diffstat:
 fs/xfs/libxfs/xfs_ag.c |   93 ++++---------------------------------------------
 fs/xfs/libxfs/xfs_ag.h |   14 -------
 fs/xfs/xfs_icache.c    |   77 ++++++++++++++++++++++++++--------------
 fs/xfs/xfs_mount.h     |    3 -
 fs/xfs/xfs_super.c     |    3 -
 fs/xfs/xfs_trace.h     |    3 -
 include/linux/xarray.h |    1 
 lib/xarray.c           |   33 +++++++++++++++++
 8 files changed, 96 insertions(+), 131 deletions(-)