generic: ENOSPC regression test in a multi-threaded scenario

generic: ENOSPC regression test in a multi-threaded scenario

Test allocation strategies of the file system and validate space
anomalies as reported by the system versus the allocated by the
program.

The test is motivated by a bug in ext4 systems where-in ENOSPC is
reported by the file system even though enough space for allocations is
available[1].

[1]: https://patchwork.ozlabs.org/patch/1294003

Linux kernel patch series that fixes the above regression:
53f86b170dfa ("ext4: mballoc: add blocks to PA list under same spinlock
              after allocating blocks")
cf5e2ca6c990 ("ext4: mballoc: refactor ext4_mb_discard_preallocations()")
07b5b8e1ac40 ("ext4: mballoc: introduce pcpu seqcnt for freeing PA to
              improve ENOSPC handling")
8ef123fe02ca ("ext4: mballoc: refactor ext4_mb_good_group()")
993778306e79 ("ext4: mballoc: use lock for checking free blocks while
              retrying")

Suggested-by: Ritesh Harjani <riteshh@linux.ibm.com>
Co-authored-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Pratik Rajesh Sampat <psampat@linux.ibm.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>