libquota: cleanup libquota code

libquota: cleanup libquota code

This patch cleans up the quota code as suggested in previous reviews. This
includes
* remove BUG_ON()s and 'exit()' calls from library code
* remove calls to malloc/free and instead use ext2fs_get/free_mem functions.
* lib/quota/common.c file in not needed anymore and is removed.
* rename exported functions to start with quota_
  (ex: init_quota_context --> quota_init_context)
* better error handling in quota library

Signed-off-by: Aditya Kali <adityakali@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>