LU-17173 utils: cleanup lfs flushctx

LU-17173 utils: cleanup lfs flushctx

When lfs flushctx is called without mount points, build the list of
all mounts first, and then call the ioctl to flush associated
contexts. Otherwise fetching the mount points unfortunately refreshes
the contexts being flushed, because the mount points are being
accessed.

Lustre-change: https://review.whamcloud.com/52604
Lustre-commit: f0534544e3e3aef280ccc5f042e37d42d33b28d3

Test-Parameters: trivial
Test-Parameters: kerberos=true testlist=sanity-krb5
Test-Parameters: testgroup=review-dne-selinux-ssk-part-2
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I75b9efe4c65ce66f5f692f9e49a28fde705d0140
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53909
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>