btrfs: add helper for bit enumeration

Define helper macro that can be used in enum {} to utilize the automatic
increment to define all bits without directly defining the values or
using additional linear bits.

1. capture the sequence value, N
2. use the value to define the given enum with N-th bit set
3. reset the sequence back to N

Use for enums that do not require fixed values for symbolic names (like
for on-disk structures):

enum {
	ENUM_BIT(FIRST),
	ENUM_BIT(SECOND),
	ENUM_BIT(THIRD)
};

Where the values would be 0x1, 0x2 and 0x4.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>