NFS: On fatal writeback errors, we need to call nfs_inode_remove_request()

If the writeback error is fatal, we need to remove the tracking structures
(i.e. the nfs_page) from the inode.

Fixes: 6fbda89b

 ("NFS: Replace custom error reporting mechanism...")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>