fs: add atomic write unit max opt to statx

XFS will be able to support large atomic writes (atomic write > 1x block)
in future. This will be achieved by using different operating methods,
depending on the size of the write.

Specifically a new method of operation based in FS atomic extent remapping
will be supported in addition to the current HW offload-based method.

The FS method will generally be appreciably slower performing than the
HW-offload method. However the FS method will be typically able to
contribute to achieving a larger atomic write unit max limit.

XFS will support a hybrid mode, where HW offload method will be used when
possible, i.e. HW offload is used when the length of the write is
supported, and for other times FS-based atomic writes will be used.

As such, there is an atomic write length at which the user may experience
appreciably slower performance.

Advertise this limit in a new statx field, stx_atomic_write_unit_max_opt.

When zero, it means that there is no such performance boundary.

Masks STATX{_ATTR}_WRITE_ATOMIC can be used to get this new field. This is
ok for older kernels which don't support this new field, as they would
report 0 in this field (from zeroing in cp_statx()) already. Furthermore
those older kernels don't support large atomic writes - apart from block
fops, but there would be consistent performance there for atomic writes
in range [unit min, unit max].

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Acked-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>

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