ext2: Correct maximum ext2 filesystem block size

Ext2 has traditionally supported filesystem block sizes upto page size
or upto 65536. Macro EXT2_MAX_BLOCK_SIZE is set to 4096, however that is
never used in ext2 so practically we always allowed whatever
sb_set_blocksize() accepted. Fix value of EXT2_MAX_BLOCK_SIZE because it
will be used in the next patch.

Signed-off-by: Jan Kara <jack@suse.cz>