block/raw-posix: use seek_hole ahead of fiemap

block/raw-posix: use seek_hole ahead of fiemap

try_fiemap() uses FIEMAP_FLAG_SYNC which has a significant performance
impact.

Prefer seek_hole() over fiemap() to avoid this impact where possible.
seek_hole is more widely used and, arguably, has potential to be
optimised in the kernel.

Reported-By: Michael Steffens <[email protected]>
Signed-off-by: Tony Breeds <[email protected]>
Cc: Kevin Wolf <[email protected]>
Cc: Markus Armbruster <[email protected]>
Cc: Stefan Hajnoczi <[email protected]>
Cc: Max Reitz <[email protected]>
Cc: Pádraig Brady <[email protected]>
Cc: Eric Blake <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>