erofs: fix out-of-bound access when z_erofs_gbuf_growsize() partially fails

In the Linux kernel, the following vulnerability has been resolved:

erofs: fix out-of-bound access when z_erofs_gbuf_growsize() partially fails

If z_erofs_gbuf_growsize() partially fails on a global buffer due to

memory allocation failure or fault injection (as reported by syzbot [1]),

new pages need to be freed by comparing to the existing pages to avoid

memory leaks.

However, the old gbuf->pages[] array may not be large enough, which can

lead to null-ptr-deref or out-of-bound access.

Fix this by checking against gbuf->nrpages in advance.

[1] https://lore.kernel.org/r/[email protected]