Fix file system racing and ext2 directory insertion (#58)

* fix: ext2 directory insertion; racing on inode create

use nesting lock for v_inode and v_dnode locking.

hold the lock on the parent directory upon creating inode
prevent contention across multiple process that partakes
similar actions

fix the edge case of dirent insertion with data block
boundary crossing.

fix the incorrect calculation between fsblock index and
ext2 data block/inode index.

new test case: fragfile - random file spammer

* add locks to the shared objects of ext2 fs

* fix missing locks in the vfs subsystem

* add locks to dnode and inode cache
* add locks to fdtable for resolving inter-threads contentions

* scale up the fragfile sample size