nvmet: fix a memory leak in controller identify

Simply free an allocated buffer once we copied its content
to the request sgl.

kmemleak complaint:
unreferenced object 0xffff8cd40c388000 (size 4096):
  comm "kworker/2:2H", pid 14739, jiffies 4401313113
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace (crc 0):
    [<ffffffff9e01087a>] kmemleak_alloc+0x4a/0x90
    [<ffffffff9d30324a>] __kmalloc_cache_noprof+0x35a/0x420
    [<ffffffffc180b0e2>] nvmet_execute_identify+0x912/0x9f0 [nvmet]
    [<ffffffffc181a72c>] nvmet_tcp_try_recv_pdu+0x84c/0xc90 [nvmet_tcp]
    [<ffffffffc181ac02>] nvmet_tcp_io_work+0x82/0x8b0 [nvmet_tcp]
    [<ffffffff9cfa7158>] process_one_work+0x178/0x3e0
    [<ffffffff9cfa8e9c>] worker_thread+0x2ec/0x420
    [<ffffffff9cfb2140>] kthread+0xf0/0x120
    [<ffffffff9cee36a4>] ret_from_fork+0x44/0x70
    [<ffffffff9ce7fdda>] ret_from_fork_asm+0x1a/0x30

Fixes: 84909f7d ("nvmet: use kzalloc instead of ZERO_PAGE in nvme_execute_identify_ns_nvm()")
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>