LU-921 test: drop subtest 24 from ALWAYS_EXCEPT

LU-921 test: drop subtest 24 from ALWAYS_EXCEPT

a new subtest 24a is introduced, drop it from ALWAYS_EXCEPT
to allow it to be tested, and verify whether the newer linux
kernel handle fsync(by filemap_fdatawait) correctly.

Pls see BZ5494 to more detail.

Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Change-Id: I88d5c6870827cafc5534aba33661b0c8180a8d5c
Reviewed-on: http://review.whamcloud.com/4443
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>