lockfile: Fix hang locking through a dangling symlink.

lockfile: Fix hang locking through a dangling symlink.

open() with O_CREAT|O_EXCL yields EEXIST if the file being opened is a
symlink.  lockfile_try_lock() interpreted that error code to mean that
some other process had created the lock file in the meantime, so it went
around its loop again, which found out the same thing, which led to a hang.

This commit fixes the problem by dropping O_EXCL.  I don't see any reason
that it's actually necessary.  That means that the loop itself is
unnecessary, so this commit drops that too.

Debian bug #681880.
CC: 681880@bugs.debian.org
Reported-by: Bastian Blank <waldi@debian.org>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Reviewed-by: Simon Horman <horms@verge.net.au>