Kernel: Simplify the File memory-mapping API

Before this change, we had File::mmap() which did all the work of
setting up a VMObject, and then creating a Region in the current
process's address space.

This patch simplifies the interface by removing the region part.
Files now only have to return a suitable VMObject from
vmobject_for_mmap(), and then sys$mmap() itself will take care of
actually mapping it into the address space.

This fixes an issue where we'd try to block on I/O (for inode metadata
lookup) while holding the address space spinlock. It also reduces time
spent holding the address space lock.

Kernel/FileSystem/File.cpp

@@ -35,7 +35,7 @@ ErrorOr<void> File::ioctl(OpenFileDescription&, unsigned, Userspace<void*>)
     return ENOTTY;
 }
 
-ErrorOr<Memory::Region*> File::mmap(Process&, Memory::AddressSpace&, OpenFileDescription&, Memory::VirtualRange const&, u64, int, bool)
+ErrorOr<NonnullLockRefPtr<Memory::VMObject>> File::vmobject_for_mmap(Process&, Memory::VirtualRange const&, u64&, bool)
 {
     return ENODEV;
 }