bus/pci: fix mapping BAR containing MSI-X table

bus/pci: fix mapping BAR containing MSI-X table

When the BAR contains MSI-X table, pci_vfio_mmap_bar() tries to skip
the table and map the rest. "map around it" is the phrase used in the
source. The function splits the BAR into two regions: the region
before the table (first part or memreg[0]) and the region after the
table (second part or memreg[1]).

For hardware that has MSI-X vector table offset 0, the first part does
not exist (memreg[0].size == 0).

  Capabilities: [60] MSI-X: Enable- Count=48 Masked-
         Vector table: BAR=2 offset=00000000
         PBA: BAR=2 offset=00001000

The mapping part of the function maps the first part, if it
exists. Then, it maps the second part, if it exists and "if mapping the
first part succeeded".

The recent change that replaces MAP_FAILED with NULL breaks the "if
mapping the first part succeeded" condition (1) in the snippet below.

    void *map_addr = NULL;
    if (memreg[0].size) {
	    /* actual map of first part */
	    map_addr = pci_map_resource(...);
    }

    /* if there's a second part, try to map it */
    if (map_addr != NULL                              // -- (1)
	    && memreg[1].offset && memreg[1].size) {
	[...]
    }

    if (map_addr == NULL) {
            RTE_LOG(ERR, EAL, "Failed to map pci BAR%d\n",
                    bar_index);
            return -1;
    }

When the first part does not exist, (1) sees map_addr is still NULL,
and the function fails. This behavior is a regression and fails
probing hardware with vector table offset 0.

Previously, (1) was "map_addr != MAP_FAILED", which meant
pci_map_resource() was actually attempted and failed. So, expand (1)
to check if the first part exists as well, to match the semantics of
MAP_FAILED.

Bugzilla ID: 539
Fixes: e200535c1ca3 ("mem: drop mapping API workaround")

Signed-off-by: Hyong Youb Kim <hyonkim@cisco.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>