fs/dcache.c: avoid panic while lockref of dentry overflow

euler inclusion
category: bugfix
bugzilla: 14351
CVE: NA
---------------------------

We use lockref for dentry reference without notice that so many negative
dentry under one dir can lead to overflow of lockref. This can lead to
system crash if we do this under root dir.

Since there is not a perfect solution, we just limit max number of
dentry count up to INT_MAX / 2. Also, it will cost a lot of time from
INT_MAX / 2 to INT_MAX, so we no need to do this under protection of
dentry lock.

Also, we limit the FILES_MAX to INT_MAX / 2, since a lot open for
same file can lead to overflow too.

Changelog:
v1->v2: add a function to do check / add a Macro to mean INT_MAX / 2

Signed-off-by: yangerkun <yangerkun@huawei.com>
Reviewed-by: Miao Xie <miaoxie@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>