Kernel: Add command line option readonly

This prevents calls to write_sectors_impl and all dirty pages are always
kept in RAM.

kernel/kernel/Storage/DiskCache.cpp

@@ -1,3 +1,4 @@
+#include <kernel/BootInfo.h>
 #include <kernel/Lock/LockGuard.h>
 #include <kernel/Memory/Heap.h>
 #include <kernel/Memory/PageTable.h>
@@ -105,6 +106,9 @@ namespace Kernel
 
 	BAN::ErrorOr<void> DiskCache::sync()
 	{
+		if (g_disable_disk_write)
+			return {};
+
 		for (auto& cache : m_cache)
 		{
 			if (cache.dirty_mask == 0)

kernel/kernel/Storage/StorageDevice.cpp

@@ -2,6 +2,7 @@
 #include <BAN/ScopeGuard.h>
 #include <BAN/StringView.h>
 #include <BAN/UTF8.h>
+#include <kernel/BootInfo.h>
 #include <kernel/FS/DevFS/FileSystem.h>
 #include <kernel/FS/VirtualFileSystem.h>
 #include <kernel/Lock/LockGuard.h>
@@ -265,6 +266,13 @@ namespace Kernel
 
 		LockGuard _(m_mutex);
 
+		if (g_disable_disk_write)
+		{
+			if (!m_disk_cache.has_value())
+				return BAN::Error::from_errno(EIO);
+			return m_disk_cache->write_to_cache(lba, buffer, true);
+		}
+
 		if (!m_disk_cache.has_value())
 			return write_sectors_impl(lba, sector_count, buffer);