arm64: mte: Optimize mte_assign_mem_tag_range()

mte_assign_mem_tag_range() is called on production KASAN HW hot
paths. It makes sense to optimize it in an attempt to reduce the
overhead.

Optimize mte_assign_mem_tag_range() based on the indications provided at
[1].

[1] https://bit.ly/39AC0if



Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>