RFR(xxs): 8203680: os::stat() on Posix platform does not need to copy input path

[Thomas Stüfe]

Hi,

may I get reviews for this tiny cleanup/fix.

Bug: https://bugs.openjdk.java.net/browse/JDK-8203680
patch: http://cr.openjdk.java.net/~stuefe/webrevs/8203680-os-stat-posix-should-not-create-copy/webrev.00/webrev/

os::stat() on all Posix platforms copies the input path into a fixed
sized temp buffer, thereby risking truncation, only to call
os::native_path() which is a noop on all Posix platforms.

This patch replaces this with a straight call to stat(2), so no
truncation anymore. Also, it unifies the coding in os_posix.cpp.

Thanks, Thomas