- trivial: cache file->f_dentry->d_inode; saves a few bytes of compiled
   size.
 - move expand_stack inside ARCH_STACK_GROWSUP, add an alternate
   implementation for PA-RISC.
 - partially fix the comment (mmap_sem is held for READ, not for WRITE).
   It still doesn't make sense, saying we don't need to take the spinlock
   right before we take it.  I expect one of the vm hackers will know
   what the right thing is.