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Description of problem:
When using the -f flag and stderr is directed to /dev/null if the umount syscall
fails, umount believes it succeded and removes the entry from /etc/mtab.

Version-Release number of selected component (if applicable):
mount-2.11r-10

How reproducible:
Always

Steps to Reproduce:
1. Mount a nfs share at /mnt/test
2. 'cd /mnt/test' so that share is not umountable
3. 'umount -f /mnt/test 2>/dev/null' (stderr must be redirected)
	

Actual Results:  The share is still mounted (as expected) but there is no entry
in /etc/mtab for it.

Expected Results:  The umount command should fail and not modify the /etc/mtab file

Additional info:

This occurs because errno is not saved across a perror() call. umount2() is
called and fails (because mount point is busy), then perror() fails (because
stderr redirected to /dev/null) and modifies errno, yet errno is consulted as
being the error code of the failed umount2() call.

The stderr redirected to /dev/null occurs in all initscripts since commands are
run through initlog. It bites particularly hard in '/etc/init.d/netfs stop'. So
it's important to fix.

This was already reported back in Dec-2001 for RedHat Linux 7.1, but seems to
have been ignored since then. See bug #57286