RDMA/rxe: Fix mr->map double free

In the Linux kernel, the following vulnerability has been resolved:

RDMA/rxe: Fix mr->map double free

rxe_mr_cleanup() which tries to free mr->map again will be called when
rxe_mr_init_user() fails:

CPU: 0 PID: 4917 Comm: rdma_flush_serv Kdump: loaded Not tainted 6.1.0-rc1-roce-flush+ #25
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
Call Trace:
<TASK>
dump_stack_lvl+0x45/0x5d
panic+0x19e/0x349
end_report.part.0+0x54/0x7c
kasan_report.cold+0xa/0xf
rxe_mr_cleanup+0x9d/0xf0 [rdma_rxe]
__rxe_cleanup+0x10a/0x1e0 [rdma_rxe]
rxe_reg_user_mr+0xb7/0xd0 [rdma_rxe]
ib_uverbs_reg_mr+0x26a/0x480 [ib_uverbs]
ib_uverbs_handler_UVERBS_METHOD_INVOKE_WRITE+0x1a2/0x250 [ib_uverbs]
ib_uverbs_cmd_verbs+0x1397/0x15a0 [ib_uverbs]

This issue was firstly exposed since commit b18c7da63fcb ("RDMA/rxe: Fix
memory leak in error path code") and then we fixed it in commit
8ff5f5d9d8cf ("RDMA/rxe: Prevent double freeing rxe_map_set()") but this
fix was reverted together at last by commit 1e75550648da (Revert
"RDMA/rxe: Create duplicate mapping tables for FMRs")

Simply let rxe_mr_cleanup() always handle freeing the mr->map once it is
successfully allocated.