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.