ocfs2: Set inline xattr entries with ocfs2_xa_set()

ocfs2_xattr_ibody_set() is the only remaining user of
ocfs2_xattr_set_entry().  ocfs2_xattr_set_entry() actually does two
things: it calls ocfs2_xa_set(), and it initializes the inline xattrs.
Initializing the inline space really belongs in its own call.

We lift the initialization to ocfs2_xattr_ibody_init(), called from
ocfs2_xattr_ibody_set() only when necessary.  Now
ocfs2_xattr_ibody_set() can call ocfs2_xa_set() directly.
ocfs2_xattr_set_entry() goes away.

Another nice fact is that ocfs2_init_dinode_xa_loc() can trust
i_xattr_inline_size.

Signed-off-by: Joel Becker <joel.becker@oracle.com>