arm64: mm: Simplify check in arch_kfence_init_pool()

arm64: mm: Simplify check in arch_kfence_init_pool()

TL;DR: checking force_pte_mapping() in arch_kfence_init_pool() is
sufficient

Commit ce2b3a50ad92 ("arm64: mm: Don't sleep in
split_kernel_leaf_mapping() when in atomic context") recently added
an arm64 implementation of arch_kfence_init_pool() to ensure that
the KFENCE pool is PTE-mapped. Assuming that the pool was not
initialised early, block splitting is necessary if the linear
mapping is not fully PTE-mapped, in other words if
force_pte_mapping() is false.

arch_kfence_init_pool() currently makes another check: whether
BBML2-noabort is supported, i.e. whether we are *able* to split
block mappings. This check is however unnecessary, because
force_pte_mapping() is always true if KFENCE is enabled and
BBML2-noabort is not supported. This must be the case by design,
since KFENCE requires PTE-mapped pages in all cases. We can
therefore remove that check.

The situation is different in split_kernel_leaf_mapping(), as that
function is called unconditionally regardless of the configuration.
If BBML2-noabort is not supported, it cannot do anything and bails
out. If force_pte_mapping() is true, there is nothing to do and it
also bails out, but these are independent checks.

Commit 53357f14f924 ("arm64: mm: Tidy up force_pte_mapping()")
grouped these checks into a helper, split_leaf_mapping_possible().
This isn't so helpful as only split_kernel_leaf_mapping() should
check both. Revert the parts of that commit that introduced the
helper, reintroducing the more accurate comments in
split_kernel_leaf_mapping().

Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>