mountd: don't require mountpoint in crossmnt case

mountd: don't require mountpoint in crossmnt case

Currently,

	mount --bind /path /path

where /path is a subdirectory of a crossmnt export, can cause client
hangs, since the kernel detects that as a mountpoint, but nfs-util's
is_mountpoint() function does not.

I don't see any sure-fire way to detect such mountpoints.  But that's
OK: it's harmless to allow this upcall to succeed even when the
directory is not a mountpoint, so let's just remove this check.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>