[PATCH] Make fdescfs MPSAFE

Hi,

To be able to better understand VFS and locking in general, I started making
fdescfs MPSAFE. I'm not experienced with any of these things, so there might be
some errors, although I've looked through much VFS code and code for other FS
like nullfs. I've tested it by running two pthreads on the same fd, and that seamt
to work, but there might be other cases where it will fail.

Patch is attached.
-- 
Ulf Lilleengen