LU-255: enable ext4 features by default

LU-255: enable ext4 features by default

Enable the flex_bg, huge_file, and dir_nlink features from ext4 by
default for ext4-based ldiskfs filesystems.  The flex_bg feature
can significantly reduce e2fsck time as well as time-to-first-write
on newly mounted OSTs.

Reduce the number of inodes created on larger OST filesystems, since
there are far too many inodes created by default, which wastes space
and slows down e2fsck.

Increase the number of inodes on MDT filesystems, subject to
constraints from the default number of stripes being stored for
each inode, which may increase the inode size being used.  Remove
changes to the statfs() results on the MDT to limit inode count to
the returned block count since increased MDT inodes will always
exceed free blocks, and it was confusing.

Skip zeroing the journal at format time.  It will be overwritten
at first use, and avoids writing 400MB of zeroes to the filesystem
needlessly.  We can't skip zering the inode table by default, but
newer mke2fs will check if the kernel handles this itself, and
request it internally.

Cleanups to remove support for 2.4 kernels and reiserfs filesystem.

Change-Id: I519bffaad5e97ee68c189efb00e07ebc0b1600f5
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/480
Reviewed-by: Yu Jian <yujian@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>