ocfs2: Set the xattr name+value pair in one place

We create two new functions on ocfs2_xa_loc, ocfs2_xa_prepare_entry()
and ocfs2_xa_store_inline_value().

ocfs2_xa_prepare_entry() makes sure that the xl_entry field of
ocfs2_xa_loc is ready to receive an xattr.  The entry will point to an
appropriately sized name+value region in storage.  If an existing entry
can be reused, it will be.  If no entry already exists, it will be
allocated.  If there isn't space to allocate it, -ENOSPC will be
returned.

ocfs2_xa_store_inline_value() stores the data that goes into the 'value'
part of the name+value pair.  For values that don't fit directly, this
stores the value tree root.

A number of operations are added to ocfs2_xa_loc_operations to support
these functions.  This reflects the disparate behaviors of xattr blocks
and buckets.

With these functions, the overlapping ocfs2_xattr_set_entry_local() and
ocfs2_xattr_set_entry_normal() can be replaced with a single call
scheme.
Signed-off-by: Joel Becker <joel.becker@oracle.com>