fs: ubifs: Ensure buf is freed before return

fs: ubifs: Ensure buf is freed before return

Returning directly after buf has been allocated will result in a memory
leak. Instead set the error code and goto the common unwind code to
ensure that buf will be freed before returning.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>