gen_init_cpio.c: fix buffer allocation size

[watan104]

Fix potential buffer overflow by allocating space for null terminator in hardlink name buffer.

[watan104]

Here's the issue I found: malloc(strlen(line)) allocates space based on the input line length, but the code then parses and reformats that data into a different structure with null-terminated strings.
The original line gets transformed from:
file1 /path 0755 0 0 hardlink1 hardlink2
into:
file1\0hardlink1\0hardlink2\0
Depending on parsing, this reformatted data can exceed the originally allocated buffer size, leading to potential buffer overflow.
The +1 ensures we have adequate space for the null terminator and accounts for the data structure transformation during hardlink processing.
This addresses a potential security vulnerability in the initramfs generation tool.