vn_getf/vn_releasef should not accept negative file descriptors

vn_getf/vn_releasef should not accept negative file descriptors

C type coercion rules require that negative numbers be converted into
positive numbers via wraparound such that a negative -1 becomes a
positive 1. This causes vn_getf to return a file handle when it should
return NULL whenever a positive file descriptor existed with the same
value. We should check for a negative file descriptor and return NULL
instead.

This was caught by ClusterHQ's unit testing.

Reference:
http://stackoverflow.com/questions/50605/signed-to-unsigned-conversion-in-c-is-it-always-safe

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Andriy Gapon <avg@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #450