USB: gadgetfs: Fix a potential memory leak in 'dev_config()'

'kbuf' is allocated just a few lines above using 'memdup_user()'.
If the 'if (dev->buf)' test fails, this memory is never released.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>