btrfs: remove check for NULL fs_info at btrfs_folio_end_lock_bitmap()

Smatch complains about possibly dereferencing a NULL fs_info at
btrfs_folio_end_lock_bitmap():

  fs/btrfs/subpage.c:332 btrfs_folio_end_lock_bitmap() warn: variable dereferenced before check 'fs_info' (see line 326)

because we access fs_info to set the 'start_bit' variable before doing the
check for a NULL fs_info.

However fs_info is never NULL, since in the only caller of
btrfs_folio_end_lock_bitmap() is extent_writepage(), where we have an
inode which always as a non-NULL fs_info.

So remove the check for a NULL fs_info at btrfs_folio_end_lock_bitmap().

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