BUG: Fix/restore broken NFS double mount test

[stephensmalley]

tests/nfs_filesystem: remove failing mount
For reasons unclear, we attempt to mount twice, the 2nd time yields EBUSY,
and then the following test fails. Remove the 2nd mount, which also
resolves the failure.

Output:
Run NFS context specific tests
nfs_filesystem/test .. 2/56 creat(2) Failed: Permission denied

Failed test at nfs_filesystem/test line 118.

nfs_filesystem/test .. 30/56 creat(2) Failed: Permission denied

Failed test at nfs_filesystem/test line 118.

nfs_filesystem/test .. 53/56 # Looks like you failed 2 tests of 56.
nfs_filesystem/test .. Dubious, test returned 2 (wstat 512, 0x200)
Failed 2/56 subtests

Test Summary Report

nfs_filesystem/test (Wstat: 512 (exited 2) Tests: 56 Failed: 2)
Failed tests: 7, 35
Non-zero exit status: 2
Files=1, Tests=56, 12 wallclock secs ( 0.02 usr 0.00 sys + 0.15 cusr 0.25 csys = 0.42 CPU)
Result: FAIL
Failed 1/1 test programs. 2/56 subtests failed.
Error on line: 104 - Closing down NFS
umount: /mnt/selinux-testsuite: not mounted.
NFS Closed down

[stephensmalley]

This appears to be a return of an older kernel bug described in the test script comments:
# Mount a security_label exported NFS filesystem twice, confirm that
# NFS security labeling support isn't silently disabled by trying to
# set a label on a file and confirm it is set.
# Fixed by kernel commit 3815a245b50124f0865415dcb606a034e97494d4
# "security/selinux: fix SECURITY_LSM_NATIVE_LABELS on reused superblock"
#