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Description of problem:
For browsable autofs maps, a mount request that arrives at the same time 
an expire is happening can fail to perform the needed mount.

This happens becuase the directory exists and so the revalidate succeeds 
when we need it to fail so that lookup is called on the same dentry to do 
the mount. Instead lookup is called on the next path component which 
should be whithin the mount, but the parent isn't mounted. 

The solution is to allow the revalidate to continue and perform the mount 
as no directory creation (at mount time) is needed for browsable mount 
entries.


Version-Release number of selected component (if applicable):


How reproducible:
Sometimes

Steps to Reproduce:
I put together the following reproducer for this:

auto.master:
/multi	/etc/auto.multi --timeout=4 --ghost

and auto.multi contains
dir     /       segfault:/export/multi \
        /foo    segfault:/export/multi/thefoodir

Then, I simply run the following script:

#!/bin/bash
while true;
do
	cat /segfault/multi/foo/bar > /dev/null
	if [ $? -ne 0 ]; then
		echo "Failed!"
		exit 1
	fi
	sleep 4
done

Once we get a mount request at the same time as a pending expiry, we will
trigger this bug.  In my testing, this typically occurs withing the first 3
iterations of the loop.

Additional info: