fix(fs-lib): append root fstab entry only when missing

[prabhakarpujeri]

Problem

Commit b12f818 ("feat(nbd): support ipv6 link local nbds") changed the previously unconditional append in write_fs_tab() into a conditional — to avoid duplicate entries — but inverted the test:

if grep -q "$_root /sysroot" /etc/fstab; then
    echo "$_root /sysroot ..." >> /etc/fstab
else
    return
fi

As a result:

• Normal case (entry absent): the function returns early and the /sysroot entry is never added to the initramfs fstab.
• Already-present case (e.g. a iscsi/nbd hook runs twice): the entry is appended again, creating a duplicate.

Callers in the iscsi and nbd modules (iscsiroot.sh, parse-iscsiroot.sh, nbdroot.sh) rely on write_fs_tab() to record the root filesystem with the resolved rootfstype/rootflags and to trigger daemon-reload + initrd-root-fs.target.

Fix

Negate the test so the entry is appended exactly when it is missing, and drop the now-pointless else return. This preserves the original de-duplication intent of b12f818.

Testing

Sandboxed harness executing the real write_fs_tab() (old from git show HEAD:, new from the working tree), with /etc/fstab repointed to a scratch file:

case	before	after
entry missing	nothing written	one correct entry (auto ro,x-initrd.mount 0 0)
entry pre-seeded	duplicate appended	still exactly one entry
rw on cmdline	—	ro,x-initrd.mount,rw

shellcheck modules.d/70fs-lib/fs-lib.sh clean.

Fixes: b12f818 ("feat(nbd): support ipv6 link local nbds")

[devkontrol]

CC @mwilck