Flash API: `flash_erase` has incorrect userspace handler

[de-nordic]

Describe the bug

Drivers have option to not implement erase, in which case pointer to erase callback may be left NULL:

zephyr/include/zephyr/drivers/flash.h

Lines 309 to 312 in c14b022

	static inline int z_impl_flash_erase(const struct device *dev, off_t offset,
	size_t size)
	{
	int rc = -ENOSYS;

But handler for the erase has been left as it is always expected:

zephyr/drivers/flash/flash_handlers.c

Line 34 in c14b022

K_OOPS(K_SYSCALL_DRIVER_FLASH(dev, erase));

Expected behavior
Proper check should be done using K_SYSCALL_OBJ, to only verify the object, ignoring whether erase callback is assigned or not.

K_OOPS(K_SYSCALL_OBJ(dev, K_OBJ_DRIVER_FLASH))

Impact
Currently only on out-of-tree drivers that do not implement erase.

Environment (please complete the following information):

• OS: Ubuntu 20
• Toolchain Zephyr sdk 0.17.0
• Commit SHA c14b022

Additional context
Verification Return Value Policies
K_SYSCALL_OBJ
K_SYSCALL_DRIVER_OP