include/linux/fs.h: fix unsigned enum warning with gcc-4.2

include/linux/fs.h: fix unsigned enum warning with gcc-4.2

With arm-linux-gcc-4.2, almost every file we build in the kernel ends up
with this warning:

  include/linux/fs.h:2648: warning: comparison of unsigned expression < 0 is always false

Later versions don't have this problem, but it's easy enough to work
around.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnd Bergmann <[email protected]>
Cc: Russell King <[email protected]>
Cc: Brendan Gregg <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>