btrfs: fix some error checking for btrfs_decompress()

The btrfs_decompress() function mostly (u32)-1 on error but it can
also return -EPERM or other kernel error codes from zstd_decompress().
The "ret" variable is an int, so we could just check for negatives.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Qu Wenruo <wqu@suse.com>