ext4: fix potential memory leak in ext4_fc_record_modified_inode()

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

ext4: fix potential memory leak in ext4_fc_record_modified_inode()

As krealloc may return NULL, in this case 'state->fc_modified_inodes'

may not be freed by krealloc, but 'state->fc_modified_inodes' already

set NULL. Then will lead to 'state->fc_modified_inodes' memory leak.