libext2fs: reject various bitmap and inode operations for journal_dev file systems

libext2fs: reject various bitmap and inode operations for journal_dev file systems

The ext2fs_open() function will only allow journal_dev file systems to
be open if explicitly requested by programs using the
EXT2_FLAG_JOURNAL_DEV_OK flag.  Those programs will not try to call
functions that make no sense, such as ext2fs_read_inode(),
ext2fs_read_bitmaps(), etc.  Just to make things the library more
robust against buggy programs (or unrealistic fuzzers) add a check for
journal_dev file systems to various ext2fs library functions to return
a new error, EXT2_ET_EXTERNAL_JOURNAL_NOSUPP.

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