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792fdbbb13
The kernel has not required a 16 MiB limit on dedupe requests since
v4.18-rc1 b67287682688 ("Btrfs: dedupe_file_range ioctl: remove 16MiB
restriction").
Kernels before v4.18 would truncate the request and return the size
actually deduped in `bytes_deduped`. Kernel v4.18 and later will loop
in the kernel until the entire request is satisfied (although still
in 16 MiB chunks, so larger extents will be split).
Modify the loop in userspace to measure the size the kernel actually
deduped, instead of assuming the kernel will only accept 16 MiB.
On current kernels this will always loop exactly once.
Since we now rely on `bytes_deduped`, make sure it has a sane value.
Signed-off-by: Zygo Blaxell <bees@furryterror.org>