btrfs: split btrfs_fs_closing() and change return type to bool

btrfs: split btrfs_fs_closing() and change return type to bool

There are two tests in btrfs_fs_closing() but checking the
BTRFS_FS_CLOSING_DONE bit is done only in one place
load_extent_tree_free(). As this is an inline we can reduce size of the
generated code. The types can be also changed to bool as this becomes a
simple condition.

   text    data     bss     dec     hex filename
1674006  146704   15560 1836270  1c04ee pre/btrfs.ko
1673772  146704   15560 1836036  1c0404 post/btrfs.ko

DELTA: -234

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>