vfs_btrfs: fix compile on 32-bit platforms.

vfs_btrfs: fix compile on 32-bit platforms.

uint64_t are not unsigned longs on 32-bit platforms:

[3265/3996] Compiling source3/modules/vfs_btrfs.c
../source3/modules/vfs_btrfs.c: In function ‘btrfs_copy_chunk_send’:
../source3/modules/vfs_btrfs.c:118:3: error: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 3 has type ‘uint64_t’ [-Werror=format]
../source3/modules/vfs_btrfs.c:118:3: error: format ‘%ld’ expects argument of type ‘long int’, but argument 4 has type ‘int64_t’ [-Werror=format]
../source3/modules/vfs_btrfs.c:118:3: error: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 5 has type ‘uint64_t’ [-Werror=format]
../source3/modules/vfs_btrfs.c:118:3: error: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 7 has type ‘uint64_t’ [-Werror=format]
../source3/modules/vfs_btrfs.c: In function ‘btrfs_copy_chunk_recv’:
../source3/modules/vfs_btrfs.c:180:2: error: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 2 has type ‘off_t’ [-Werror=format]
cc1: some warnings being treated as errors

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>