smb: client: drain and invalidate before server-side copy/clone

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

smb: client: drain and invalidate before server-side copy/clone

cifs_file_copychunk_range() and the clone (FICLONE) path of
cifs_remap_file_range() do not serialise the destination page cache
against the server-side copy the way the other server-side range
operations (smb3_zero_range(), smb3_punch_hole(), smb3_collapse_range()
and smb3_insert_range()) do.

cifs_file_copychunk_range() invalidates the destination range with
filemap_invalidate_inode(), which takes and drops the mapping's
invalidate_lock internally, so the lock is no longer held when the
copychunk ioctl is issued. It also never drains in-flight netfs I/O on
the target. The clone path never takes the invalidate_lock at all (only
i_rwsem), uses a bare truncate_inode_pages_range() and likewise does not
drain outstanding I/O.

As a result an asynchronous destination writeback can complete after the
server-side copy/clone has run and reinstate stale data over the region
just written by the server, corrupting the file. This is the same class
of corruption as commit d7d2adcd022b ("smb/client: flush dirty data
before punching a hole") and has been seen randomly in generic/363
against Windows Server.

Fix both paths to follow the established ordering: hold the target
mapping's invalidate_lock across the flush and invalidation of the
destination and the server ioctl, and call netfs_wait_for_outstanding_io()
on the target to drain in-flight writes before the ioctl is issued. Only
the target inode's invalidate_lock is required, as the source is merely
flushed and not invalidated; i_rwsem (already held via
lock_two_nondirectories()) is acquired before the invalidate_lock,
matching the VFS lock ordering. Since filemap_invalidate_inode() takes
that same lock internally, replace it with its own unmap/flush/invalidate
steps instead of calling it, and return early on a zero-length copy to
avoid a range underflow.