netfs: Fix a few minor bugs in netfs_page_mkwrite()

We can't return with VM_FAULT_SIGBUS | VM_FAULT_LOCKED; the core
code will not unlock the folio in this instance.  Introduce a new
"unlock" error exit to handle this case.  Use it to handle
the "folio is truncated" check, and change the "writeback interrupted
by a fatal signal" to do a NOPAGE exit instead of letting the core
code install the folio currently under writeback before killing the
process.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20241005182307.3190401-3-willy@infradead.org


Signed-off-by: Christian Brauner <brauner@kernel.org>