uapi: rename ext2_swab() to swab() and share globally in swab.h

ext2_swab() is defined locally in lib/find_bit.c However it is not
specific to ext2, neither to bitmaps.

There are many potential users of it, so rename it to just swab() and
move to include/uapi/linux/swab.h

ABI guarantees that size of unsigned long corresponds to BITS_PER_LONG,
therefore drop unneeded cast.

Link: http://lkml.kernel.org/r/20200103202846.21616-1-yury.norov@gmail.com
Signed-off-by: Yury Norov <yury.norov@gmail.com>
Cc: Allison Randal <allison@lohutok.net>
Cc: Joe Perches <joe@perches.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/uapi/linux/swab.h

@@ -4,6 +4,7 @@
 
 #include <linux/types.h>
 #include <linux/compiler.h>
+#include <asm/bitsperlong.h>
 #include <asm/swab.h>
 
 /*
@@ -132,6 +133,15 @@ static inline __attribute_const__ __u32 __fswahb32(__u32 val)
 	__fswab64(x))
 #endif
 
+static __always_inline unsigned long __swab(const unsigned long y)
+{
+#if BITS_PER_LONG == 64
+	return __swab64(y);
+#else /* BITS_PER_LONG == 32 */
+	return __swab32(y);
+#endif
+}
+
 /**
  * __swahw32 - return a word-swapped 32-bit value
  * @x: value to wordswap