libext2fs: add metadata checksum and snapshot feature flags

libext2fs: add metadata checksum and snapshot feature flags

Reserve EXT4_FEATURE_RO_COMPAT_METADATA_CSUM and
EXT2_FEATURE_COMPAT_EXCLUDE_BITMAP.  Also reserve fields in the
superblock and the inode for the checksums.  In the block group
descriptor, reserve the exclude bitmap field for the snapshot feature,
and checksums for the inode and block allocation bitmaps.

With this commit, the metadata checksum and exclude bitmap features
should have reserved all of the fields they need in ext4's on-disk
format.

This commit also fixes an a missing byte swap for s_overhead_blocks.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Darrick J. Wong <djwong@us.ibm.com>
Cc: Amir Goldstein <amir73il@gmail.com>