fix(dracut-install): protect against broken links pointing to themselves

readlink does not return an error if a symbolic link points to itself, which can cause a stack overflow due to infinite recursion in the get_real_file function.

Although this type of recursive links should not exist, we discovered this issue on a real system. It can be reproduced as follows:

> ls -l /lib64/libblkid.so
-rwxr-xr-x 1 root root 224368 Aug  9 15:13 /lib64/libblkid.so
> rm -f /lib64/libblkid.so
> ln -s /lib64/libblkid.so /lib64/libblkid.so
> ls -l /lib64/libblkid.so
lrwxrwxrwx 1 root root 18 Aug  9 15:06 /lib64/libblkid.so -> /lib64/libblkid.so
> dracut -f -I "/lib64/libblkid.so" test.img
...
dracut-install: Handle '/lib64/libblkid.so'
dracut-install: dracut_install('/lib64/libblkid.so', '/lib64/libblkid.so', 0, 0, 1)
dracut-install: get_real_file('/lib64/libblkid.so')
dracut-install: get_real_file: readlink('/lib64/libblkid.so') returns '/lib64/libblkid.so'
dracut-install: get_real_file('/lib64/libblkid.so') => '/lib64/libblkid.so'
...
[infinite recursion]
...
dracut-install: dracut_install('/lib64/libblkid.so', '/lib64/libblkid.so', 0, 0, 1)
dracut-install: get_real_file('/lib64/libblkid.so')
dracut-install: get_real_file: readlink('/lib64/libblkid.so') returns '/lib64/libblkid.so'
dracut-install: get_real_file('/lib64/libblkid.so') => '/lib64/libblkid.so'
dracut-install: dracut_install('/lib64/libblkid.so', '/lib64/libblkid.so', 0, 0, 1)
/usr/lib/dracut/dracut-init.sh: line 298: 20949 Segmentation fault      (core dumped) $DRACUT_INSTALL ${dracutsysrootdir:+-r "$dracutsysrootdir"} ${initdir:+-D "$initdir"} -a ${loginstall:+-L "$loginstall"} ${DRACUT_RESOLVE_DEPS:+-l} ${DRACUT_FIPS_MODE:+-f} ${_hostonly_install:+-H} "$@"
dracut: FAILED: /usr/lib/dracut/dracut-install --debug -D /var/tmp/dracut.dqLmOS/initramfs -a /lib64/libblkid.so
...

After applying this patch:

> dracut -f -I "/lib64/libblkid.so" test.img
...
dracut-install: Handle '/lib64/libblkid.so'
dracut-install: dracut_install('/lib64/libblkid.so', '/lib64/libblkid.so', 0, 0, 1)
dracut-install: get_real_file('/lib64/libblkid.so')
dracut-install: get_real_file: readlink('/lib64/libblkid.so') returns '/lib64/libblkid.so'
dracut-install: ERROR: '/lib64/libblkid.so' is pointing to itself.
dracut-install: ERROR: installing '/lib64/libblkid.so'
dracut: FAILED: /usr/lib/dracut/dracut-install --debug -D /var/tmp/dracut.4w8FVL/initramfs -a /lib64/libblkid.so
...

Checklist

[X] I have tested it locally
[ ] I have reviewed and updated any documentation if relevant
[ ] I am providing new code and test(s) for it

dracutdevs / dracut

fix(dracut-install): protect against broken links pointing to themselves #2478

Checklist