virtiofsd: Add help for -o xattr-mapping

The option is not documented in help.

Add small help about the option.

Signed-off-by: Carlos Venegas <jose.carlos.venegas.munoz@intel.com>
Message-Id: <20210414201207.3612432-3-jose.carlos.venegas.munoz@intel.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Connor Kuehl <ckuehl@redhat.com>

tools/virtiofsd/helper.c

@@ -172,6 +172,9 @@ void fuse_cmdline_help(void)
            "                               default: no_writeback\n"
            "    -o xattr|no_xattr          enable/disable xattr\n"
            "                               default: no_xattr\n"
+           "    -o xattrmap=<mapping>      Enable xattr mapping (enables xattr)\n"
+           "                               <mapping> is a string consists of a series of rules\n"
+           "                               e.g. -o xattrmap=:map::user.virtiofs.:\n"
            "    -o modcaps=CAPLIST         Modify the list of capabilities\n"
            "                               e.g. -o modcaps=+sys_admin:-chown\n"
            "    --rlimit-nofile=<num>      set maximum number of file descriptors\n"