vfs: Require specification of size of mount data for internal mounts

vfs: Require specification of size of mount data for internal mounts

Require specification of the size of the mount data passed to the VFS
mounting functions by internal mounts.  The problem is that the legacy
handling for the upcoming mount-context patches has to copy an entire page
as that's how big the buffer is defined as being, but some of the internal
calls pass in a short bit of stack space, with the result that the stack
guard page may get hit.

Signed-off-by: David Howells <dhowells@redhat.com>