Rework r348303 to reduce the time of holding global BPF lock.

It appeared that using NET_EPOCH_WAIT() while holding global BPF lock
can lead to another panic:

spin lock 0xfffff800183c9840 (turnstile lock) held by 0xfffff80018e2c5a0 (tid 100325) too long
panic: spin lock held too long
...
#0 sched_switch (td=0xfffff80018e2c5a0, newtd=0xfffff8000389e000, flags=<optimized out>) at /usr/src/sys/kern/sched_ule.c:2133
#1 0xffffffff80bf9912 in mi_switch (flags=256, newtd=0x0) at /usr/src/sys/kern/kern_synch.c:439
#2 0xffffffff80c21db7 in sched_bind (td=<optimized out>, cpu=<optimized out>) at /usr/src/sys/kern/sched_ule.c:2704
#3 0xffffffff80c34c33 in epoch_block_handler_preempt (global=<optimized out>, cr=0xfffffe00005a1a00, arg=<optimized out>)

at /usr/src/sys/kern/subr_epoch.c:394

#4 0xffffffff803c741b in epoch_block (global=<optimized out>, cr=<optimized out>, cb=<optimized out>, ct=<optimized out>)

at /usr/src/sys/contrib/ck/src/ck_epoch.c:416

#5 ck_epoch_synchronize_wait (global=0xfffff8000380cd80, cb=<optimized out>, ct=<optimized out>) at /usr/src/sys/contrib/ck/src/ck_epoch.c:465
#6 0xffffffff80c3475e in epoch_wait_preempt (epoch=0xfffff8000380cd80) at /usr/src/sys/kern/subr_epoch.c:513
#7 0xffffffff80ce970b in bpf_detachd_locked (d=0xfffff801d309cc00, detached_ifp=<optimized out>) at /usr/src/sys/net/bpf.c:856
#8 0xffffffff80ced166 in bpf_detachd (d=<optimized out>) at /usr/src/sys/net/bpf.c:836
#9 bpf_dtor (data=0xfffff801d309cc00) at /usr/src/sys/net/bpf.c:914

To fix this add the check to the catchpacket() that BPF descriptor was
not detached just before we acquired BPFD_LOCK().

Reported by: slavash
Tested by: slavash
MFC after: 1 week