LU-18813 osd-wbcfs: MemFS-based OSD with writeback support

LU-18813 osd-wbcfs: MemFS-based OSD with writeback support

Implement a memory filesystem based OSD with writeback support for
Lustre.
It borrows lots of design from memory-based file systems such as
tmpfs/ramfs. The data is first written into the memory-based file
system (called MemFS for short). And then, the data can be flushed
to the persistent storage in a delayed write-back manner.

This patch implemented the basic functionality to store data in
MemFS. It can reuse lots of VFS codes in Linux kernel such as:
- Page caching for data;
- dcache for dentry management and lookup;
- icache for inode management and lookup;
- Writeback mechanism in Linux kernel

Test-Parameters: testlist=sanity fstype=wbcfs mdscount=1 mdtcount=1 osscount=4 ostcount=1
Test-Parameters: testlist=sanity fstype=wbcfs mdscount=1 mdtcount=1 osscount=4 ostcount=1
Test-Parameters: testlist=sanity fstype=wbcfs mdscount=4 mdtcount=1 osscount=4 ostcount=1
Test-Parameters: testlist=sanity fstype=wbcfs mdscount=4 mdtcount=1 osscount=4 ostcount=1
Test-Parameters: testlist=sanity fstype=wbcfs combinedmdsmgs=false standalonemgs=true mdscount=1 mdtcount=1 osscount=4 ostcount=1
Test-Parameters: testlist=sanity fstype=wbcfs combinedmdsmgs=false standalonemgs=true mdscount=1 mdtcount=1 osscount=4 ostcount=1
Test-Parameters: testlist=sanity fstype=wbcfs combinedmdsmgs=false standalonemgs=true mdscount=4 mdtcount=1 osscount=4 ostcount=1
Test-Parameters: testlist=sanity fstype=wbcfs combinedmdsmgs=false standalonemgs=true mdscount=4 mdtcount=1 osscount=4 ostcount=1
Signed-off-by: Yingjin Qian <qian@ddn.com>
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: Ia07c1d95b7ad3f7f5e817a8de69d0a4ab6995ffa
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58439
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>