MFC: r333579

MFC: r333579
The NFSv4.1 server should return NFSERR_BACKCHANBUSY instead of NFS_OK.

When an NFSv4.1 session is busy due to a callback being in progress,
nfsrv_freesession() should return NFSERR_BACKCHANBUSY instead of NFS_OK.
The only effect this has is that the DestroySession operation will report
the failure for this case and this probably has little or no effect on a
client. Spotted by inspection and no failures related to this have been
reported.

git-svn-id: svn://svn.freebsd.org/base/stable/10@336234 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f