openvswitch: fix OOB access in reserve_sfa_size()

commit cefa91b2 upstream.

Given a sufficiently large number of actions, while copying and
reserving memory for a new action of a new flow, if next_offset is
greater than MAX_ACTIONS_BUFSIZE, the function reserve_sfa_size() does
not return -EMSGSIZE as expected, but it allocates MAX_ACTIONS_BUFSIZE
bytes increasing actions_len by req_size. This can then lead to an OOB
write access, especially when further actions need to be copied.

Fix it by rearranging the flow action size check.

KASAN splat below:

==================================================================
BUG: KASAN: slab-out-of-bounds in reserve_sfa_size+0x1ba/0x380 [openvswitch]
Write of size 65360 at addr ffff888147e4001c by task handler15/836

CPU: 1 PID: 836 Comm: handler15 Not tainted 5.18.0-rc1+ #27
...
Call Trace:
 <TASK>
 dump_stack_lvl+0x45/0x5a
 print_report.cold+0x5e/0x5db
 ? __lock_text_start+0x8/0x8
 ? reserve_sfa_size+0x1ba/0x380 [openvswitch]
 ka...