kernel: k_work: k_work_init() should initialize all fields

k_work_init() was not initializing all fields in the k_work struct.

Mainly, the atomic_clear_bit() function call was reading a possibly
uninitialized value, clearing a bit, and assigning it back to the
`flags` member.  The `_reserved` member was never initialized.

With the struct now initialized with the _K_WORK_INITIALIZER() macro,
initialization is consistent regardless of how a `struct k_work` is
initialized.

This fixes the Valgrind issues found in #7478.

Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>