This prevents a double-fetch from user space that can lead to to an
undersized allocation and heap overflow.
Fixes: 54dbc1517237 ("vfs: hoist the btrfs deduplication ioctl to the vfs") Signed-off-by: Scott Bauer <sbauer@plzdonthack.me> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>