Call sysctl_head_finish on error

This error path returned directly instead of calling sysctl_head_finish().

Fixes: ef9d965b ("sysctl: reject gigantic reads/write to sysctl files")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>