e2fsprogs: only update the icache for ext2_inode

e2fsprogs: only update the icache for ext2_inode

We only read the cache when:

bufsize == sizeof(struct ext2_inode)

then we should only update the cache in the same condition, otherwise
there would be errors, for example:

cache[0]: cached ino 14 when bufsize = 128 by ext2fs_write_inode_full()
cache[1]: cached ino 14 when bufsize = 156 by ext2fs_read_inode_full()

Then update the cache:
cache[0]: cached ino 15 when bufsize = 156 by ext2fs_read_inode_full()

Then the ino 14 would hit the cache[1] when bufsize = 128 (but it was
cached by bufsize = 156), so there would be errors.

[YOCTO #3848]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>