xen-gntalloc: introduce missing kfree

Error handling code following a kmalloc should free the allocated data.
Out_unlock is used on both success and failure, so free vm_priv before
jumping to that label.

A simplified version of the semantic match that finds the problem is as
follows: (http://coccinelle.lip6.fr)

// <smpl>
@r exists@
local idexpression x;
statement S;
identifier f1;
position p1,p2;
expression *ptr != NULL;
@@

x@p1 = \(kmalloc\|kzalloc\|kcalloc\)(...);
...
if (x == NULL) S
<... when != x
     when != if (...) { <+...x...+> }
x->f1
...>
(
 return \(0\|<+...x...+>\|ptr\);
|
 return@p2 ...;
)

@script:python@
p1 << r.p1;
p2 << r.p2;
@@

print "* file: %s kmalloc %s return %s" % (p1[0].file,p1[0].line,p2[0].line)
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
[v1: Altered the description a bit]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>