LU-5252 hsm: make path argument to fifo event a const

LU-5252 hsm: make path argument to fifo event a const

Neither llapi_hsm_register_event_fifo() nor
llapi_hsm_unregister_event_fifo() take a const as their argument.
This leads to the following compilation warning when a caller is
using a const char *:

  ... warning: passing argument 1 of ‘llapi_hsm_register_event_fifo’
         discards ‘const’ qualifier from pointer target type
         [enabled by default]
   rc = llapi_hsm_register_event_fifo(lustre_fifo_event);
   ^
  ... note: expected ‘char *’ but argument is of type ‘const char *’
  extern int llapi_hsm_register_event_fifo(char *path);

Making path a const fixes this warning.

This is a backport to b2_5

Lustre-change://review.whamcloud.com/10807
Lustre-commit: 8264927b92b82e5ec6d223523db8680ee8df1da4

Change-Id: Ia2544cafefef8d5d1e4cf4cfc6408aac373c447b
Signed-off-by: frank zago <fzago@cray.com>
Reviewed-on: http://review.whamcloud.com/10807
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
(cherry picked from commit 8264927b92b82e5ec6d223523db8680ee8df1da4)
Reviewed-on: http://review.whamcloud.com/11363