We now use AK::Error and AK::ErrorOr<T> in both kernel and userspace!
This was a slightly tedious refactoring that took a long time, so it's
not unlikely that some bugs crept in.
Nevertheless, it does pass basic functionality testing, and it's just
real nice to finally see the same pattern in all contexts. :^)
And also try_create<T> => try_make_ref_counted<T>.
A global "create" was a bit much. The new name matches make<T> better,
which we've used for making single-owner objects since forever.
The quickmap_page() and unquickmap_page() functions are used to map a
single physical page at a kernel virtual address for temporary access.
These use the per-CPU quickmap buffer in the page tables, and access to
this is guarded by the MM lock. To prevent bugs, quickmap_page() should
not *take* the MM lock, but rather verify that it is already held!
This exposed two situations where we were using quickmap without holding
the MM lock during page fault handling. This patch is forced to fix
these issues (which is great!) :^)
This makes for nicer handling of errors compared to checking whether a
RefPtr is null. Additionally, this will give way to return different
types of errors in the future.
