IB/core: Eliminate sparse false context imbalance warning

When write_gid function needs to do a sleep-able operation, it unlocks
table->rwlock and then relocks it. Sparse complains about context
imbalance.

This is safe as write_gid is always called with table->rwlock.
write_gid protects from simultaneous writes to this GID entry
by setting the GID_TABLE_ENTRY_INVALID flag.

Fixes: 9c584f04 ('IB/core: Change per-entry lock in RoCE GID table to
		     one lock')
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>