Improve handling of crossing FSAL boundaries in POSIX namespace

Take the following filesystems:

/export/xfs
/export/xfs/some/path/ext4

Assume /export/xfs is exported FSAL_XFS and /export/xfs/some/path/ext4
is exported FSAL_VFS. The directory /export/xfs/some/path/ext4 must
have two cache inodes, one that looks like an FSAL_XFS inode, and one
that looks like an FSAL_VFS inode. These are required to make the
export junction work.

Of course xfs can not do name_to_handle_at on
/export/xfs/some/path/ext4. When we get an error back from that
call, we just invent a handle (basically just the fsid wich will
not conflict with any FSAL_VFS or FSAL_XFS handle.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>