Monitor: improve check_one_sharer() for checking duplicated process

Monitor: improve check_one_sharer() for checking duplicated process

When running mdadm monitor with scan mode, only one autorebuild process
is allowed. check_one_sharer() checks duplicated process by following
steps,
1) Read autorebuild.pid file,
   - if file does not exist, no duplicated process, go to 3).
   - if file exists, continue to next step.
2) Read pid number from autorebuild.pid file, then check procfs pid
   directory /proc/<PID>,
   - if the directory does not exist, no duplicated process, go to 3)
   - if the directory exists, print error message for duplicated process
     and exit this mdadm.
3) Write current pid into autorebuild.pid file, continue to monitor in
   scan mode.

The problem for the above step 2) is, if after system reboots and
another different process happens to have exact same pid number which
autorebuild.pid file records, check_one_sharer() will treat it as a
duplicated mdadm process and returns error with message "Only one
autorebuild process allowed in scan mode, aborting".

This patch tries to fix the above same-pid-but-different-process issue
by one more step to check the process command name,
1) Read autorebuild.pid file
   - if file does not exist, no duplicated process, go to 4).
   - if file exists, continue to next step.
2) Read pid number from autorebuild.pid file, then check procfs file
   comm with the specific pid directory /proc/<PID>/comm
   - if the file does not exit, it means the directory /proc/<PID> does
     not exist, go to 4)
   - if the file exits, continue next step
3) Read process command name from /proc/<PIC>/comm, compare the command
   name with "mdadm" process name,
   - if not equal, no duplicated process, goto 4)
   - if strings are equal, print error message for duplicated process
     and exit this mdadm.
4) Write current pid into autorebuild.pid file, continue to monitor in
   scan mode.

Now check_one_sharer() returns error for duplicated process only when
the recorded pid from autorebuild.pid exists, and the process has exact
same command name as "mdadm".

Reported-by: Shinkichi Yamazaki <shinkichi.yamazaki@suse.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>