debugfs: add 64-bit support to the set_field commands

debugfs: add 64-bit support to the set_field commands

The set_fields commands (set_super_value, set_inode_field,
set_block_group) now handle fields which store in split fields on
ext4's on-disk format.  For example, the superblock fields
s_blocks_count and s_blocks_count_hi.

The user can either set the low or high part of the field via
"blocks_count_lo" or "blocks_count_hi", or both parts can be set via
"blocks_count".

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>