Invalid inode cache in nffs_write_append()

[yashi]

;; Even though README says we must report to Jira, I found that Jira is deprecated. So here I go.

nffs_write_append() gets a pointer to struct nffs_cache_inode as the first argument. Unfortunately, the function calls nffs_block_entry_reserve() as the first thing in the the function. Because nffs_block_entry_reserve() might invalidate the given cache in low memory situation, subsequent accesses to the cache must be protected by nffs_cache_inode_ensure().