Documentation/filesystems/vfs.txt: update file_operations documentation

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This describes how the VFS can manipulate an open file. As of kernel

3.5, the following members are defined:

3.12, the following members are defined:

struct file_operations {

	struct module *owner;

	int (*aio_fsync) (struct kiocb *, int datasync);

	int (*fasync) (int, struct file *, int);

	int (*lock) (struct file *, int, struct file_lock *);

	ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);

	ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);

	ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *);

	ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);

	unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);

	int (*check_flags)(int);

	ssize_t (*splice_read)(struct file *, struct pipe_inode_info *, size_t, unsigned int);

	int (*setlease)(struct file *, long arg, struct file_lock **);

	long (*fallocate)(struct file *, int mode, loff_t offset, loff_t len);

	int (*show_fdinfo)(struct seq_file *m, struct file *f);

};

Again, all methods are called without any locks being held, unless

  lock: called by the fcntl(2) system call for F_GETLK, F_SETLK, and F_SETLKW

  	commands

  readv: called by the readv(2) system call

  writev: called by the writev(2) system call

  sendfile: called by the sendfile(2) system call

  get_unmapped_area: called by the mmap(2) system call

  check_flags: called by the fcntl(2) system call for F_SETFL command