libbb: add xfstat function

function                                             old     new   delta
xfstat                                                 -      25     +25
mkfs_ext2_main                                      2421    2423      +2
mkfs_reiser_main                                    1197    1194      -3
next                                                 312     307      -5
ar_main                                              533     522     -11
mkfs_minix_main                                     2938    2924     -14
mkfs_vfat_main                                      1511    1495     -16
writeTarFile                                         272     255     -17
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/6 up/down: 27/-66)            Total: -39 bytes

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
diff --git a/include/libbb.h b/include/libbb.h
index 43e525c..6fb0438 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -411,6 +411,7 @@
 void bb_unsetenv_and_free(char *key) FAST_FUNC;
 void xunlink(const char *pathname) FAST_FUNC;
 void xstat(const char *pathname, struct stat *buf) FAST_FUNC;
+void xfstat(int fd, struct stat *buf, const char *errmsg) FAST_FUNC;
 int xopen(const char *pathname, int flags) FAST_FUNC;
 int xopen_nonblocking(const char *pathname) FAST_FUNC;
 int xopen3(const char *pathname, int flags, int mode) FAST_FUNC;