Preliminary work to hook up fiemap, this allows us to pass in an
arbitrary formatter to copy extent data back to userspace.

The formatter takes info for 1 extent, a pointer to the user "thing*"
and a pointer to a "filled" variable to indicate whether a userspace
buffer did get filled in (for fiemap, hole "extents" are skipped).

I'm just using the getbmapx struct as a "common denominator" because
as far as I can see, it holds all info that any formatters will care
about.

("*thing" because fiemap doesn't pass the user pointer around, but rather
has a pointer to a fiemap info structure, and helpers associated with it)
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Niv Sardi <xaiki@sgi.com>