USB: gadget: f_mass_storage: fix in error recovery

In to places in fsg_common_init() an unconditional call to kfree()
on common was performed in error recovery which is not a valid
behaviour since fsg_common structure is not always allocated by
fsg_common_init().

To fix, the calls has been replaced with a goto to a proper error
recovery which does the correct thing.

Also, refactored fsg_common_release() function.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Reviewed-by: Viral Mehta <viral.mehta@lntinfotech.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>