maple_tree: Remove mas_walk in favour of mas_load.

maple_tree: Remove mas_walk in favour of mas_load.

mas_walk was called in a single place for loading the value.  Rename the
function and clean it up.  Add retry on dead node detection and retry
entries.  Note that this can return the zero entry.

Also fix comments for _mas_walk

Also update limits on _mas_walk so that the maple state can be used for
checking the range limit of a search result.

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>