e2fsck: add support for xattrs in external inodes

e2fsck: add support for xattrs in external inodes

Add support for the INCOMPAT_EA_INODE feature, which stores large
extended attributes into an external inode instead of data blocks.
The inode is referenced by the e_value_inum field (formerly the
unused e_value_block field) from the extent header, and stores the
xattr data starting at byte offset 0 in the inode data block.

The xattr inode stores the referring inode number in its i_mtime,
and the parent i_generation in its own i_generation, so that there
is a solid linkage between the two that e2fsck can verify.  The
xattr inode is itself marked with EXT4_EA_INODE_FL as well.

Signed-off-by: Kalpak Shah <kalpak@clusterfs.com>
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>