Fix double free in case of mount error.

[fsu]

Summary

It was found, that if cg descriptors bread() will return error, the fs->e2fs_gd and fs->e2fs_contigdirs sb fields will be freed twice.
Immediately after bread() call and at the bottom of the ext2_mountfs() function.
It is not possible to free these in-memory superblocks fields immediately after bread(), because the compute_sb_data() function is used in case of ro/rw remounting.
So, choose the second place and remove free() calls immediately after bread().

Test Plan

Tested manually with problem fs image.
And some ro/rw remount manual tests had place.

[pfg]

sys/fs/ext2fs/ext2_vfsops.c

474

Remove "The" from here: we are talking about two things so we cannot use a singular.