Fix missing EXPORT_SYMBOL_GPL() statement for mas_preallocate().

It isn't actually used by anything yet, but mas_preallocate() is part of
the maple tree's 'Advanced API'.  All other functions of this API are
exported already.

Link: https://lkml.kernel.org/r/20230302011035.4928-1-dakr@redhat.comSigned-off-by: Danilo Krummrich <dakr@redhat.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>