Btrfs: fix snprintf usage by send's gen_unique_name

Btrfs: fix snprintf usage by send's gen_unique_name

The buffer size argument passed to snprintf must account for the
trailing null byte added by snprintf, and it returns a value >= then
sizeof(buffer) when the string can't fit in the buffer.

Since our buffer has a size of 64 characters, and the maximum orphan
name we can generate is 63 characters wide, we must pass 64 as the
buffer size to snprintf, and not 63.

Signed-off-by: Filipe David Borba Manana <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: Josef Bacik <[email protected]>
Signed-off-by: Chris Mason <[email protected]>