vfs: relax dependency of close() performance on rcu grace periods

Currently, we perform the final fdrop() in an rcu callback, which means it
can have considerable latency.  This in turn places restrictions on
further optimizations we can do to rcu, as we need prompt close() execution.

Relax this by performing the fdrop() immediately, and only deferring the
final free().  To do this, we harden fget() so it now expects files with
positive refcounts, and make sure that after the final fdrop() refcounts
are either zero or negative.

This also makes close() take effect immediately, which fixes tst-pipe.so.