erofs: Fix EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS default logic

When NR_CPUS is less than 16, or when SMP is disabled, the default value
of 16 is invalid.

While actual configuration picks up a sensible and valid default
(NR_CPUS or 1), "make savedefconfig" will still write a line like

    CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS=1

to the defconfig file, even if that matches the sensible default.

Avoid needlessly enlarging the defconfig files, and reduce churn for
updating them, by specifying valid defaults depending on SMP and
NR_CPUS.

While at it, make the prompt depend on SMP, as there is no point in
asking the user about the maximum number of decompression streams if
there is only one valid answer.

Fixes: c9b47e6b23114e93 ("erofs: cap LZMA stream pool size")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <xiang@kernel.org>