[PATCH] uml ubd driver: do not store error codes as ->fd

To simplify error handling, make sure fd is saved into ubd_dev->fd only when
we are sure it is an fd and not an error code.

Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>