fs_mgr: report errno string for __mount errors

Need to know why the mount failed.  clang_format adjustment.

Basically change LINFO to PINFO to cause the log message for the mount
report to be accompanied by a strerror(errno) message appended to the
end so that it is clear why the mount was rejected.

Test: manual
Bug: 63100799
Change-Id: Ic958299759befe5d5b11bdc95fea5d64cad86412
diff --git a/fs_mgr/fs_mgr.cpp b/fs_mgr/fs_mgr.cpp
index 8c19a81..ba30756 100644
--- a/fs_mgr/fs_mgr.cpp
+++ b/fs_mgr/fs_mgr.cpp
@@ -476,10 +476,11 @@
         if ((info.st_mode & S_IFMT) == S_IFLNK)
             unlink(target);
     mkdir(target, 0755);
+    errno = 0;
     ret = mount(source, target, rec->fs_type, mountflags, rec->fs_options);
     save_errno = errno;
-    LINFO << __FUNCTION__ << "(source=" << source << ",target="
-          << target << ",type=" << rec->fs_type << ")=" << ret;
+    PINFO << __FUNCTION__ << "(source=" << source << ",target=" << target
+          << ",type=" << rec->fs_type << ")=" << ret;
     if ((ret == 0) && (mountflags & MS_RDONLY) != 0) {
         fs_mgr_set_blk_ro(source);
     }