checkpatch: don't require kernel style __attribute__ shortcuts in uapi paths

Avoid prescribing kernel styled shortcuts for gcc extensions of
__attribute__((foo)) in the uapi include paths.

Fix $realfile filename when using -f/--file to not remove first level
directory as if the filename was used in a -P1 patch.  Only strip the
first level directory (typically a or b) for P1 patches.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: "Dixit, Ashutosh" <ashutosh.dixit@intel.com>
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Git-commit: 2b7ab45395dc4d91ef30985f76d90a8f28f58c27
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>