LU-15536 utils: add lfs somsync utility

LU-15536 utils: add lfs somsync utility

This patch adds lfs somsync utility to synchronize
SOM xattr(s) for given FILE(s) or FID(s).

lfs somsync FILE ...
lfs somsync --by-fid MOUNT FID ...

Test-Parameters: trivial env=ONLY=807 testlist=sanity

Change-Id: Ie9ee39625d56ec026c89dcc0f27025904ca354e3
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46942
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>