You'll usually want /usr/src/linux to be a symlink to the actual sources for the kernel you're using, i.e. /usr/src/linux-<version>.
It makes it simple in case you keep mutiple sources/kernel.
So, if you switch kernel or add a new one, you just delete the old link,
rm /usr/src/linux
and create a new one to the kernel you want to use/compile
ln -s /usr/src/linux-<version> /usr/src/linux.
This way /usr/src/linux always point to the source of the kernel in use.
HTH
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