ext2fs: Handle internal journal over 2^32 bytes

ext2fs: Handle internal journal over 2^32 bytes

The write_journal_inode() code is only setting the low 32-bit i_size
for the journal size, even though it is possible to specify a journal
up to 10M blocks in size.  Trying to create a journal larger than 2GB
will succeed, but an immediate e2fsck would fail.  Store i_size_high
for the journal inode when creating it, and load it upon access.

Use s_jnl_blocks[15] to store the journal i_size_high backup.  This
field is currently unused, as EXT2_N_BLOCKS is 15, so it is using
s_jnl_blocks[0..14], and i_size is in s_jnl_blocks[16].

Rename the "size" argument "num_blocks" for the journal creation functions
to clarify this parameter is in units of filesystem blocks and not bytes.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>