ocfs2: Allocation in ocfs2_xa_prepare_entry(), values in ocfs2_xa_store_value()

ocfs2_xa_prepare_entry() gets all the logic to add, remove, or modify
external value trees.  Now, when it exits, the entry is ready to receive
a value of any size.

ocfs2_xa_remove() is added to handle the complete removal of an entry.
It truncates the external value tree before calling
ocfs2_xa_remove_entry().

ocfs2_xa_store_inline_value() becomes ocfs2_xa_store_value().  It can
store any value.

ocfs2_xattr_set_entry() loses all the allocation logic and just uses
these functions.  ocfs2_xattr_set_value_outside() disappears.

ocfs2_xattr_set_in_bucket() uses these functions and makes
ocfs2_xattr_set_entry_in_bucket() obsolete.  That goes away, as does
ocfs2_xattr_bucket_set_value_outside() and
ocfs2_xattr_bucket_value_truncate().
Signed-off-by: Joel Becker <joel.becker@oracle.com>