ext4: add bounds checking in get_max_inline_xattr_value_size()

In the Linux kernel, the following vulnerability has been resolved:

ext4: add bounds checking in get_max_inline_xattr_value_size()

Normally the extended attributes in the inode body would have been checked when the inode is first opened, but if someone is writing to the block device while the file system is mounted, it's possible for the inode table to get corrupted. Add bounds checking to avoid reading beyond the end of allocated memory if this happens.