LU-14121 nodemap: do not force fsuid/fsgid squashing

LU-14121 nodemap: do not force fsuid/fsgid squashing

In the current implementation, if the real uid is squashed, then the
fsuid is similarly squashed, no matter what is the value of the
effective uid.
This squashing is a little bit too strict, and we should instead trust
mapped fsuid and fsgid values.

Also add euid_access test program and sanity-sec test_55 to verify
the issue is fixed.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Iecaecac5054b105cd42206b0a9a3868cde0269b4
Reviewed-on: https://review.whamcloud.com/40645
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>