fs/xattr.c: zero out memory copied to userspace in getxattr

fs/xattr.c: zero out memory copied to userspace in getxattr

getxattr uses vmalloc to allocate memory if kzalloc fails.  This is
filled by vfs_getxattr and then copied to the userspace.  vmalloc,
however, doesn't zero out the memory so if the specific implementation
of the xattr handler is sloppy we can theoretically expose a kernel
memory.  There is no real sign this is really the case but let's make
sure this will not happen and use vzalloc instead.

Fixes: 779302e67835 ("fs/xattr.c:getxattr(): improve handling of allocation failures")
Link: http://lkml.kernel.org/r/[email protected]
Acked-by: Kees Cook <[email protected]>
Reported-by: Vlastimil Babka <[email protected]>
Signed-off-by: Michal Hocko <[email protected]>
Cc: <[email protected]>	[3.6+]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>