duplicate ovl_getxattr()

ovl_getattr() returns the value of an xattr in a kmalloced buffer.  There
are two callers:

 ovl_copy_up_meta_inode_data()	(copy_up.c)
 ovl_get_redirect_xattr()	(util.c)

This patch just copies ovl_getxattr() to copy_up.c, the following patches
will deal with the differences in idividual callers.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>