fix regression on smbc_getxattr and fix doc

Fix regression introduced in the fix for https://bugzilla.samba.org/show_bug.cgi?id=14808

Documentation states:

 * @param size      The size of the buffer pointed to by value.  This parameter
 *                  may also be zero, in which case the size of the buffer
 *                  required to hold the attribute value will be returned,
 *                  but nothing will be placed into the value buffer.

But this usage is now broken (returns 0 instead of size)

Common usage is broken:

	xattr_size = smbc_getxattr(state->ctx, url, name, NULL, 0);
	values = malloc(xattr_size + 1);
	xattr_size = smbc_getxattr(state->ctx, url, name, values, xattr_size + 1);

There is no more way to retrieve the needed space.

The fix for 14808 change smbc_getxattr behavior to match documentation (@return) This fix revert this change, and fix documentation instead

As the regression appears in 4.16.9 and 4.17.5, please backport in these branches

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• (optional) This MR is just one part towards a larger feature.
• (optional, if backport required) Bugzilla bug filed and BUG: tag added
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated
• CI timeout is 3h or higher (see Settings/CICD/General pipelines/ Timeout)

Reviewer's checklist:

• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to README.Coding.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code