mmap: userswap: fix memory leak in do_mmap

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I4AHP2


CVE: NA

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When userswap is enabled, the memory pointed by 'pages' is not freed in
abnormal branch in do_mmap(). To fix the issue and keep do_mmap() mostly
unchanged, we rename do_mmap() to __do_mmap() and extract the memory
alloc and free code out of __do_mmap(). When __do_mmap() returns a error
value, we goto the error label to free the memory.

Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>