Fix possible races between umount and quota on/off.

Finally I decided to take a reference to vfsmount during vfs_quota_on() and
to drop it after the final cleanup in the vfs_quota_off().  This way we
should be all the time guarded against umount.  This way was protected also
the old code which used filp_open() for opening quota files.  I was also
thinking about other ways of protection but there would be always a window
(provided I don't want to play much with namespace locks) where
vfs_quota_on() could be called while umount() is in progress resulting in
the "Busy inodes after unmount" messages...

Get a reference to vfsmount during quotaon() so that we are guarded against
umount (as was the old code using filp_open()).
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>