CVE-2026-46040: inotify: fix watch count leak when fsnotify_add_inode_mark_locked() fails

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

inotify: fix watch count leak when fsnotifyaddinodemarklocked() fails

When fsnotifyaddinodemarklocked() fails in inotifynewwatch(), the error path calls inotifyremovefromidr() but does not call decinotifywatches() to undo the preceding incinotifywatches(). This leaks a watch count, and repeated failures can exhaust the maxuserwatches limit with -ENOSPC even when no watches are active.

Prior to commit 1cce1eea0aff ("inotify: Convert to using per-namespace limits"), the watch count was incremented after fsnotifyaddmarklocked() succeeded, so this path was not affected. The conversion moved incinotifywatches() before the mark insertion without adding the corresponding rollback.

Add the missing decinotifywatches() call in the error path.