eal/linux: fix uevent message parsing

eal/linux: fix uevent message parsing

Caught with ASan:
==9727==ERROR: AddressSanitizer: stack-buffer-overflow on address
  0x7f0daa2fc0d0 at pc 0x7f0daeefacb2 bp 0x7f0daa2fadd0 sp 0x7f0daa2fa578
READ of size 1 at 0x7f0daa2fc0d0 thread T1
    #0 0x7f0daeefacb1  (/lib64/libasan.so.5+0xbacb1)
    #1 0x115eba1 in dev_uev_parse ../lib/eal/linux/eal_dev.c:167
    #2 0x115f281 in dev_uev_handler ../lib/eal/linux/eal_dev.c:248
    #3 0x1169b91 in eal_intr_process_interrupts
  ../lib/eal/linux/eal_interrupts.c:1026
    #4 0x116a3a2 in eal_intr_handle_interrupts
  ../lib/eal/linux/eal_interrupts.c:1100
    #5 0x116a7f0 in eal_intr_thread_main
  ../lib/eal/linux/eal_interrupts.c:1172
    #6 0x112640a in ctrl_thread_init
  ../lib/eal/common/eal_common_thread.c:202
    #7 0x7f0dade27159 in start_thread (/lib64/libpthread.so.0+0x8159)
    #8 0x7f0dadb58f72 in clone (/lib64/libc.so.6+0xfcf72)

Address 0x7f0daa2fc0d0 is located in stack of thread T1 at offset 4192
  in frame
    #0 0x115f0c9 in dev_uev_handler ../lib/eal/linux/eal_dev.c:226

  This frame has 2 object(s):
    [32, 48) 'uevent'
    [96, 4192) 'buf' <== Memory access at offset 4192 overflows this
  variable
HINT: this may be a false positive if your program uses some custom
  stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
Thread T1 created by T0 here:
    #0 0x7f0daee92ea3 in __interceptor_pthread_create
  (/lib64/libasan.so.5+0x52ea3)
    #1 0x1126542 in rte_ctrl_thread_create
  ../lib/eal/common/eal_common_thread.c:228
    #2 0x116a8b5 in rte_eal_intr_init
  ../lib/eal/linux/eal_interrupts.c:1200
    #3 0x1159dd1 in rte_eal_init ../lib/eal/linux/eal.c:1044
    #4 0x7a22f8 in main ../app/test-pmd/testpmd.c:4105
    #5 0x7f0dada7f802 in __libc_start_main (/lib64/libc.so.6+0x23802)

Bugzilla ID: 792
Fixes: 0d0f478d0483 ("eal/linux: add uevent parse and process")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Tested-by: Yan Xia <yanx.xia@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>