CVE-2026-72225: jbd2: fix integer underflow in jbd2_journal_initialize_fast_commit()

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

jbd2: fix integer underflow in jbd2journalinitializefastcommit()

jbd2journalinitializefastcommit() validates journal capacity by checking (journal->jlast - numfcblks < JBD2MINJOURNALBLOCKS). Both jlast and numfcblks are unsigned, so when numfcblks exceeds jlast the subtraction wraps to a large value, bypassing the bounds check.

The resulting underflow corrupts jlast, jfcfirst, and jfree, leading to journal abort.

Fix by checking numfcblks against jlast before the subtraction, returning -EFSCORRUPTED.