libext2fs: in ext2fs_open[2](), return an error if s_desc_size is too large

libext2fs: in ext2fs_open[2](), return an error if s_desc_size is too large

Previously, ext2fs_open() and ext2fs_open2() would return an error if
s_desc_size is too small.  Add a check so it will return an error if
s_desc_size is too large, as well.

These checks will be skipped for e2fsck when it uses the flag
EXT2_FLAG_IGNORE_SB_ERRORS.

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