ext2: improve consistency of ext2_fsblk_t datatype usage

The ext2 block allocation/deallocation functions and their respective
calls use a mixture of unsigned long and ext2_fsblk_t datatypes to
index the desired ext2 block. This commit replaces occurrences of
unsigned long with ext2_fsblk_t, covering the functions
ext2_new_block(), ext2_new_blocks(), ext2_free_blocks(),
ext2_free_data() and ext2_free_branches(). This commit is rather
conservative, and only replaces unsigned long with ext2_fsblk_t if
the variable is used to index a specific ext2 block.

Signed-off-by: Georg Ottinger <g.ottinger@gmx.at>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20230817195925.10268-1-g.ottinger@gmx.at>