fs/proc/kcore.c: add mmap interface

fs/proc/kcore.c: add mmap interface

When we do the kernel monitor, use the DRGN
(https://github.com/osandov/drgn) access to kernel data structures, found
that the system calls a lot.  DRGN is implemented by reading /proc/kcore.
After looking at the kcore code, it is found that kcore does not implement
mmap, resulting in frequent context switching triggered by read.
Therefore, we want to add mmap interface to optimize performance.  Since
vmalloc and module areas will change with allocation and release,
consistency cannot be guaranteed, so mmap interface only maps KCORE_TEXT
and KCORE_RAM.

The test results:
1. the default version of kcore
real 11.00
user 8.53
sys 3.59

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
99.64  128.578319          12  11168701           pread64
...
------ ----------- ----------- --------- --------- ----------------
100.00  129.042853              11193748       966 total

2. added kcore for the mmap interface
real 6.44
user 7.32
sys 0.24

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
32.94    0.130120          24      5317       315 futex
11.66    0.046077          21      2231         1 lstat
 9.23    0.036449         177       206           mmap
...
------ ----------- ----------- --------- --------- ----------------
100.00    0.395077                 25435       971 total

The test results show that the number of system calls and time
consumption are significantly reduced.

Link: https://lkml.kernel.org/r/20210704062208.7898-1-zhoufeng.zf@bytedance.com
Co-developed-by: Ying Chen <chenying.kernel@bytedance.com>
Signed-off-by: Ying Chen <chenying.kernel@bytedance.com>
Signed-off-by: Feng Zhou <zhoufeng.zf@bytedance.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>