vfs: use READ_ONCE() to access ->i_link

Use 'READ_ONCE(inode->i_link)' to explicitly support filesystems caching
the symlink target in ->i_link later if it was unavailable at iget()
time, or wasn't easily available.  I'll be doing this in fscrypt, to
improve the performance of encrypted symlinks on ext4, f2fs, and ubifs.

->i_link will start NULL and may later be set to a non-NULL value by a
smp_store_release() or cmpxchg_release().  READ_ONCE() is needed on the
read side.  smp_load_acquire() is unnecessary because only a data
dependency barrier is required.  (Thanks to Al for pointing this out.)

Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>