shmem_notify_change and shmem_file_write be careful about overflowingly
large loff_t before shifting it into unsigned long for vm_enough_memory.
Rename SHMEM_MAX_BLOCKS to SHMEM_MAX_INDEX (to avoid confusion with
512-byte blocks), define SHMEM_MAX_BYTES from it.

But 2.5 vmtruncate lacked the s_maxbytes error handling which
shmem_notify_change now expects: bring it in from the -dj tree.
shmem_file_write error handling needs a closer look later on.