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

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/[email protected]
Signed-off-by: Yury Norov <[email protected]>
Cc: Allison Randal <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: William Breathitt Gray <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>