libext2fs: eliminate empty element holes in ext2_xattr_handle->attrs

libext2fs: eliminate empty element holes in ext2_xattr_handle->attrs

When an extended attribute is removed, its array element is emptied.
This creates holes in the array so various places that want to walk
filled elements have to do an empty element check.

Have remove operation shift remaining filled elements to the left.
This allows a simple iteration up to ext2_xattr_handle->count to walk
all filled entries, and so empty element checks become unnecessary.

Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>