fusefs: FUSE_NOTIFY_INVAL_* must busy the mountpoint

Unusually, the FUSE_NOTIFY_INVAL_INODE and FUSE_NOTIFY_INVAL_ENTRY
messages are fully asynchronous. The server sends them to the kernel
unsolicited. That means that unlike every other fuse message coming
from the server, these two arrive to a potentially unbusied mountpoint.
So they must explicitly busy it. Otherwise a page fault could result if
the mountpoint were being unmounted.

Reported by: JSML4ThWwBID69YC@protonmail.com
MFC after: 2 weeks