Xarray: remove repeat check in xas_squash_marks()

Caller of xas_squash_marks() has ensured xas->xa_sibs is non-zero.  Just
remove repeat check of xas->xa_sibs in xas_squash_marks().

Link: https://lkml.kernel.org/r/20241213122523.12764-5-shikemeng@huaweicloud.com


Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Mattew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>