posix: posix_[f]setattr() handle valid=0 case by performing {f, l}chown (-1, -1)
posix: posix_[f]setattr() handle valid=0 case by performing {f, l}chown (-1, -1)

Signed-off-by: Anand V. Avati <avati@dev.gluster.com>

BUG: 146 (Add setattr FOP)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=146

Kaushikbv/Gluster,Kaushikbv/Gluster,Kaushikbv/Gluster