btrfs: Use __u64 in exported linux/btrfs.h.

This patch fixes this build error.

/usr/include/linux/btrfs.h:121:3: error: unknown type name ‘u64’
   u64 devid;
   ^~~

Fixes: 6b526ed7

 ("btrfs: introduce device delete by devid")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Signed-off-by: David Sterba <dsterba@suse.com>