generic: test dir fsync after deleting dentry post eviction of its inode

generic: test dir fsync after deleting dentry post eviction of its inode

Test that if we fsync a file, evict its inode, unlink it and then fsync
its parent directory, after a power failure the file does not exists.

This is motivated by a bug found in btrfs, which is fixed by the following
patch for the linux kernel:

 "Btrfs: fix fsync not persisting dentry deletions due to inode evictions"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>