Increased inode block pointers to 40. The current maximum allowed (and checked).
Updates to file size after every file write ensures subsequent writes can
correctly operate using updated file size information (i.e. not try to add
more pages that are already present). We cannot do this inside write() because
directory writes rely on byte-granularity updates on file buffers, whereas
file updates are by page-granularity (currently).
Factored out mapping of the physical page as the final generic code
after all fault-specific handling is done.
Fixed the error that zero page didn't have an owner (devzero).
Fixed the error that struct dirent did not have the record length
field as u16 as expected by userspace.
Dirbuf allocation was broken, fixed it.
Added more comments on reading directories.
Issues: fs0 ought to check buf address for sys_readdir(). Currently
test0 is passing an area on its stack and fs0 is writing the data to its own
address space (i.e. an area on its own stack.)
