net/mlx5e: Replace PTP clock lock from RW lock to seq lock

Changed "priv.clock.lock" lock from 'rw_lock' to 'seq_lock'
in order to improve packet rate performance.

Tested on Intel(R) Xeon(R) CPU E5-2660 v2 @ 2.20GHz.
Sent 64b packets between two peers connected by ConnectX-5,
and measured packet rate for the receiver in three modes:
	no time-stamping (base rate)
	time-stamping using rw_lock (old lock) for critical region
	time-stamping using seq_lock (new lock) for critical region
Only the receiver time stamped its packets.

The measured packet rate improvements are:

	Single flow (multiple TX rings to single RX ring):
		without timestamping:	  4.26 (M packets)/sec
		with rw-lock (old lock):  4.1  (M packets)/sec
		with seq-lock (new lock): 4.16 (M packets)/sec
		1.46% improvement

	Multiple flows (multiple TX rings to six RX rings):
		without timestamping: 	  22   (M packets)/sec
		with rw-lock (old lock):  11.7 (M packets)/sec
		with seq-lock (new lock): 21.3 (M packets)/sec
		82.05% improvement

The packet rate improvement is due to the lack of atomic operations
for the 'readers' by the seq-lock.
Since there are much more 'readers' than 'writers' contention
on this lock, almost all atomic operations are saved.
this results in a dramatic decrease in overall
cache misses.

Signed-off-by: Shay Agroskin <shayag@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>