filemap: Fix bounds checking in filemap_read()

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

filemap: Fix bounds checking in filemap_read()

If the caller supplies an iocb->ki_pos value that is close to the

filesystem upper limit, and an iterator with a count that causes us to

overflow that limit, then filemap_read() enters an infinite loop.

This behaviour was discovered when testing xfstests generic/525 with the

"localio" optimisation for loopback NFS mounts.