ANDROID: maple_tree: Move mas_wr_modify node size calculation to mas_wr_node_size()

Create a new function to get the size of the mas_wr_node_size() since it
will be used elsewhere soon.

Drop the incrementing of the node size if this is the left-most node.

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

Link: http://git.infradead.org/users/jedix/linux-maple.git/commit/d781921bc8f9fe0720856a0956fbf3d4119a5180


[surenb: due to the differences with upstream kernel where the node size
can be obtained using mas_wr_new_end() function, this patch is not
applicable upstream. The patch was obtained from the author's tree]

Bug: 274059236
Change-Id: I9f0b5238294d0842b4c2717437ed7288b17c7617
Signed-off-by: Suren Baghdasaryan <surenb@google.com>