maple_tree: Convert forking to use the sheaf interface

Use the generic interface which should result in less bulk allocations
during a forking.

A part of this is to abstract the freeing of the sheaf or maple state
allocations into its own function so mas_destroy() and the tree
duplication code can use the same functionality to return any unused
resources.

[andriy.shevchenko@linux.intel.com: remove unused mt_alloc_bulk()]
Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>