mount/kldload race

[Jamie Gritton]

Perhaps most people don't try to mount a bunch of filesystems at the 
same time, at least not those that depend on kernel modules. But it 
turns out that's going to be a pretty common situation with jails and 
nullfs. And I found that when attempting such a feat will cause most of 
these simultaneous mounts to fail with ENODEV.

It turns out that the problem is a race in vfs_byname_kld(). First it'll 
see if the fstype is loaded, and if it isn't then it will load the 
module. But if the module is loaded by a different process between those 
two points, the resulting EEXIST from kern_kldload() will make 
vfs_byname_kld() error out.

The fix is pretty simple: don't treat EEXIST as an error. By going on, 
and rechecking for the fstype, the filesystem can be mounted while still 
allowing any "real" error to be caught. I'm including a small patch that 
will accomplish this, and I'd appreciate a quick look by anyone who's 
familiar with this part of things before I commit it.

- Jamie
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