Most users of pipe(2) do not call fstat(2) on the returned pipe descriptors.

Most users of pipe(2) do not call fstat(2) on the returned pipe descriptors.
Optimize for the case, by lazily allocating the pipe inode number at the
fstat(2) time. If alloc_unr(9) returns failure, do not fail fstat(2), since
uses of inode numbers are even rare then fstat(2), but provide zero inode
forever. Note that alloc_unr() failure is unlikely due to total number
of pipes in the system limited by the number of file descriptors.

Based on the submission by:	gianni
MFC after:	2 weeks