Replace sprintf with xsnprintf in nat/linux-osdata.c

Replace sprintf with xsnprintf in nat/linux-osdata.c

I see the following build warning when I build GDB with GCC trunk.

../../binutils-gdb/gdb/nat/linux-osdata.c: In function ‘LONGEST linux_xfer_osdata_fds(gdb_byte*, ULONGEST, ULONGEST)’:
../../binutils-gdb/gdb/nat/linux-osdata.c:767:1: error: ‘%s’ directive writing between 0 and 255 bytes into a region of size 11 [-Werror=format-length=]
 linux_xfer_osdata_fds (gdb_byte *readbuf,
 ^~~~~~~~~~~~~~~~~~~~~
../../binutils-gdb/gdb/nat/linux-osdata.c:800:51: note: format output between 7 and 262 bytes into a destination of size 17
        sprintf (procentry, "/proc/%s", dp->d_name);
                                                   ^
../../binutils-gdb/gdb/nat/linux-osdata.c: In function ‘LONGEST linux_xfer_osdata_threads(gdb_byte*, ULONGEST, ULONGEST)’:
../../binutils-gdb/gdb/nat/linux-osdata.c:555:1: error: ‘%s’ directive writing between 0 and 255 bytes into a region of size 11 [-Werror=format-length=]
 linux_xfer_osdata_threads (gdb_byte *readbuf,
 ^~~~~~~~~~~~~~~~~~~~~~~~~
../../binutils-gdb/gdb/nat/linux-osdata.c:588:51: note: format output between 7 and 262 bytes into a destination of size 17
        sprintf (procentry, "/proc/%s", dp->d_name);
                                                   ^
cc1plus: all warnings being treated as errors

The warning is a false positive, but we can workaround it by replacing
sprintf with xsnprintf.  On the other hand, it is always preferred to
use xsnprintf.

gdb:

2016-09-23  Yao Qi  <yao.qi@linaro.org>

	* nat/linux-osdata.c (linux_xfer_osdata_threads): Replace
	sprintf with xsnprintf.
	(linux_xfer_osdata_fds): Likewise.