qib_fs: fix (some) dcache abuses

* lookup_one_len() really wants i_mutex held on directory.
* leaks galore - just mount ipathfs, then
cd /sys/bus/pci/drivers/qib_ib; echo *:*:*.* >unbind
on a box with that card present and try to umount ipathfs...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>