Remove extra lstat() calls from check_mount

The os.path.exists call performs an lstat, but os.path.ismount already
performs the same check. However, it performs a separate lstat() call
to check for a symlink, which we remove as well, cutting the number
performed in half.

Sample program to be straced for comparison:

from swift.common.constraints import check_mount
import os
os.write(1, "Starting\n")
if check_mount("/", "tmp"):
    os.write(1, "Mounted\n")

Here is the output of a check on a mounted file system (common case)
using the new method:

---- strace new ----
write(1, "Starting\n", 9) = 9
lstat("/tmp", {st_mode=S_IFDIR|S_ISVTX|0777, st_size=8460, ...}) = 0
lstat("/tmp/..", {st_mode=S_IFDIR|0555, st_size=4096, ...}) = 0
write(1, "Mounted\n", 8) = 8

---- strace old ----
write(1, "Starting\n", 9) = 9
stat("/tmp", {st_mode=S_IFDIR|S_ISVTX|0777, st_size=8460, ...}) = 0
lstat("/tmp", {st_mode=S_IFDIR|S_ISVTX|0777, st_size=8460, ...}) = 0
lstat("/tmp", {st_mode=S_IFDIR|S_ISVTX|0777, st_size=8460, ...}) = 0
lstat("/tmp/..", {st_mode=S_IFDIR|0555, st_size=4096, ...}) = 0
write(1, "Mounted\n", 8) = 8

Change-Id: I027c862a2b7d9ff99d7f61bd43ccc0825dba525c
Signed-off-by: Peter Portante <peter.portante@redhat.com>
Merge "Remove extra lstat() calls from check_mount"